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Engineering 
Library 


THE  MOTOR  AND  THE  DYNAMO. 


Published  by 

The    Chemical    Publishing    Co. 

East  on,  Penna. 
Publishers  of  Scientific  Books 

Engineering  Chemistry  Portland  Cement 

Agricultural  Chemistry  Qualitative  Analysis 

M 

Household  Chemistry  Chemists'  Pocket  Manual     |j 

Metallurgy,  Etc. 


THE  MOTOR 
AND  THE  DYNAMO 


By 


JAMES  LORING  ARNOLD,  PH.D. 

PROFESSOR    OF    ELECTRICAL  ENGINEERING, 
NEW  YORK  UNIVERSITY 


EASTON,  PA. 

THE    CHEMICAL    PUBLISHING   CO. 
1913 


LONDON,    ENGLAND: 
WILLIAMS  &    NORGATE 

14    HENRIETTA  STREET,   COVENT    GARDEN,    W.    C. 


?• 7 

Library 


COPYRIGHT,  1913,  BY  EDWARD  HART. 


PREFACE. 

This  book  is  the  result  of  many  year's  experience  in  present- 
ing the  essentials  of  electrical  science  both  to  college  students 
and  practical  electricians.  It  embodies  the  substance  of  labora- 
tory conferences  and  class  room  explanations.  In  every  instance 
these  are  based  on  modern  types  of  machines,  to  the  exclusion 
of  antiquated  models.  An  unusually  large  number  of  illustra- 
tions are  inserted  for  the  purpose  of  dispensing  with  lengthy 
descriptions;  and  thanks  are  due  to  various  manufacturing  com- 
panies whose  bulletins  have  furnished  numerous  half-tones  for 
these  pages. 

The  author  hopes  that  the  book  may  be  found  practical 
and  direct  and  sufficiently  exhaustive  by  both  college  men  and 
electricians. 

J.  L.  ARNOLD. 

NEW  YORK  UNIVERSITY, 
November,  1912. 


271152 


CONTENTS 

CHAPTER  I 

INTRODUCTION 

CHAPTER  II 

MATHEMATICAL     PRINCIPLES 

Page 

(a)  Definitions 4 

(b)  The  Induced  Current 6 

(c)  Induced  Magnetic  Flux 7 

(d)  Magnetization  Curves 9 

(e)  The  Flow  of  Current I0 

CHAPTER   III 
THE  DYNAMO  MACHINE 

(a)  Frame  and  Field   Cores 17 

(b)  Field  Windings 21 

( c)  Armature  Core 23 

(d)  Armature  Windings 25 

(e)  Commutator 33 

(f)  Brushes 35 

(g)  Brush  Holders 35 

(h)  Bearings 37 

CHAPTER  IV 
OPERATION  AND  CHARACTERISTICS  OF  THE  D.  C.  DYNAMO 

(a)  Preliminary  Tests 39 

(b)  Building-up  Curve 46 

(c )  Magnetization  Curve 47 

(d)  Armature  Reaction 49 

(e)  External  Characteristics 51 

(f )  Armature  Characteristic 53 

(g)  The  Compound  Generator 54 

(h)  Sparking 56 

(i)    Operation  of  D.  C.  Shunt  Generators  in  Parallel 58 

(j)    Operation  of  D.  C.  Compound  Generators  in  Parallel 61 

(k)  D.  C.  Generators  in  Series 62 

(1)    D.  C.  Arc-light  Dynamos 64 


CONTENTS  V 

CHAPTER  V 

THE  D.  C.  MOTOR 

Page 

(a)  Operation  and  Characteristics • 66 

(b)  Varieties  of  Field  Excitation 74 

(c)  Variable  Speed  Motors 79 

(d)  Starting-boxes  and  Controllers 90 

(e)  Motor  Uses 93 

( f )  Traction  Motors 93 

(g)  The  Motor-Dynamo 97 

(h )  Losses  of  Power  in  Generators  and  Motors 98 

CHAPTER  VI 

THE  ALTERNATING  CURRENT  AND  ITS  MEASUREMENT 

(a)  The  A.  C.  Wave 105 

(b)  Mean,  Average  and  Effective  Values  • , 108 

(c)  Inductance,  or  Self-Induction no 

(d)  Capacity  in  Circuit 112 

(e)  Power  in  A.  C.  Circuits 114 

(f )  Alternating  Current  Measuring  Instruments 115 

(g)  Voltage  in  A.  C.  Circuits  in  Series 118 

(h)  Current  in  A.  C.  Circuits  in  Parallel 119 

(i)    Two-Phase  and  Three-Phase 120 

CHAPTER  VII 

ALTERNATING  CURRENT  MACHINERY 

(a)  A.  C.  Generators 124 

(b)  Voltage  Regulation  of  the  Generator 125 

(c)  The  Inductor  Alternator 129 

(d)  The  Compounding  of  Alternators 130 

(e)  The  Synchronous  Motor. 132 

(f )  Operation  of  A.  C.  Generators  in  Parallel 136 

(g)  The  Rotary  Converter 140 

(h)  The  Transformer 146 

(i)    The  Induction  Motor 150 

( j )    Starters  for  Polyphase  Induction  Motors 157 

(k)  Single-Phase  Induction  Motors 162 

(1 )    Practical  Remarks  on  Induction  Motors 167 

(m)  The  A.  C.  Series  Motor 169 


Fig.  i. — View  in  I46th  Street  Power  House,  New  York  City. 


THE  MOTOR  AND  THE  DYNAMO. 


CHAPTER  I. 

INTRODUCTION. 

In  1820  Oersted  discovered  that  a  magnetic  needle  is  affected 
by  the  presence  of  an  electric  current. — That  is,  the  electric  cur- 
rent produces  a  magnetic  field.  In  1831  Faraday  discovered  the 
induced  current. — That  is,  when  a  wire  -is  so  moved  in  a  magnetic 
field  as  to  cross  the  direction  of  the  influence  of  that  field,  an 
electro-motive  force  is  induced  in  the  wire  so  long  as  it  is  in 
motion.  On  these  two  principles  depends  the  action  of  the  mod- 
ern dynamo,  or  generator.  And  if  we  add  to-  these  Ampere's 
researches  into  the  motion  produced  by  a  magnetic  field  on  a 
current-bearing  wire,  we  shall  have  the  underlying  principle  of 
the  modern  electric  motor. 

An  apparatus  for  illustrating  these  effects  is  shown  in  Fig.  2. 


M 


Fig.  2. 

M  is  an  electro-magnet,  excited  from  some  outside  source. 
The  magnetic  field  F  is  indicated  in  the  conventional  way  by  lines 
representing  the  so-called  lines  of  force,  whose  direction  coin- 
cides with  the  directive  influence  of  the  field,  and  whose  number 
at  any  point  is  proportional  to  the  field  strength.  The  direction 
of  the  lines  of  force  outside  the  magnet  is  considered  to  be  from 
the  north  pole  to  the  south  pole  of  the  magnet. 


2  THK   MOTOR  AND  THE  DYNAMO 

If  now  the  wire  /  be  moved  vertically  downward  with  a  quick 
motion  in  direction  d  d ,  there  will  be  a  deflection  of  the  gal- 
vanometer G,  which  will  begin  to  oscillate  toward  rest  about 
its  original  position  the  instant  that  the  wire  stops  moving.  The 
more  rapid  the  motion  or  the  more  powerful  the  magnet,  the 
greater  in  a  general  way  will  be  the  deflection  of  the  galvano- 
meter. An  upward  motion  of  the  wire  (toward  d)  will  cause 
a  deflection  to  the  opposite  side,  showing  that  the  current  in  /  is 
now  in  the  reverse  direction.  If  the  wire  /  be  moved  con- 
tinually down  and  up,  or  if  it  be  given  a  rotatory  motion  so 
that  its  ends  trace  the  dotted  circles,  which  amounts  to  the  same 
thing,  there  will  be  induced  in  the  wire  an  alternating  current. 

The  wire  /  corresponds  to  one  element  of  the  winding  of  a 
drum-type  armature — the  usual  form  in  direct-current  machines. 
The  electro-magnet  corresponds  to  the  field  magnet  of  the  dyna- 
mo. The  only  thing  required  to  cause  the  current  through  the 
galvanometer  to  be  always  in  the  same  direction  is  the  inter- 
position of  a  pole-changer,  known  as  the  commutator. 

Ampere's  rule  for  the  direction  of  the  induced  current  may  be 
most  easily  expressed  by  Fig.  3,  which  represents  the  right  hand 
with  the  index  finger,  thumb,  and  middle  finger  held  at  right 


Fig.  3. — Ampere's  rule  for  induced  current. 

angles  to  one  another.     It  will  be  seen  at  once  that  a  reversal  of 
any  one  of  these  factors  necessitates  a  reversal  of  one  other. 


INTRODUCTION  3 

Conversely,  if  the  wires  be  disconnected  from  the  galvanometer 
in  Fig.  3  and  attached  to  a  source  of  current,  a  force  will  act 
between  the  current  in  wire  /  and  the  magnetic  field  in  a  direction 
perpendicular  to  both,  that  is,  parallel  to  d  d.  The  wire  /  will 
therefore  tend  to  move  upward  or  downward  according  to  the 
direction  of  the  current.  From  this  it  is  evidert  that  by  con- 
tinually reversing  the  current  sent  into  /  from  some  outside 
source,  this  wire  /  may  be  made  to  oscillate  up  and  down  across 
the  magnetic  field.  If  it  be  fastened  to  the  surface  of  a  cylinder 
whose  ends  are  represented  by  the  dotted  circles,  the  cylinder 
may  be  made  to  rotate  about  its  central  axis  by  changing  the 
direction  of  the  current  in  /  at  every  half  revolution.  For  this 
purpose  a  pole  changer  or  commutator  is  used.  The  apparatus 
then  represents  the  elemental  direct-current  motor. 

Ampere's  rule  for  the  direction  of  the  motion  of  /  in  respect 
to  the  field,  etc.,  is  the  ,same  as  in  the  preceding  case,  that  of  the 
induced  current,  except  that  the  fingers  of  the  left  hand  are  used 
instead  of  the  right. 

From  Ampere's  rules  it  can  be  seen  that  in  every  case,  the 
current,  induced  in  a  wire  by  moving  it  in  any  given  direction 
in  a  magnetic  field  is  such  that  the  force  exerted  between  this 
current  and  the  field  tends  to  push  the  wire  in  the  reverse  direc- 
tion. If  we  consider  the  resistance  of  the  wire  to  be  zero,  then 
the  force  required  to  move  the  wire  through  the  field  is  equal 
and  opposite  to  the  force  with  which  the  induced  current  in  the 
wire  opposes  this  motion.  The  converse  is  also  true.  This 
principle  follows  directly  from  the  doctrine  of  the  conservation 
of  energy  and  is  one  application  of  what  is  known  as  Lenz's  law. 

In  order  to  study  quantitatively  the  phenomena  thus  far  men- 
tioned and  to  deduce  the  fundamental  formulae  of  the  dynamo 
and  the  motor,  a  few  mathematical  considerations  are  required. 
The  next  chapter  must  be  devoted  to  this  purpose. 


CHAPTER  II. 


MATHEMATICAL  PRINCIPLES. 


(a)  Definitions. 

The  space  surrounding  a  magnet  in  which  its  influence  is  felt 
is  known  as  a  magnetic  field. 

The  field  is  usually  considered  as  made  up  of  lines  of  magnetic 
force  whose  direction  at  each  point  is  that  in  which  the  magnetic 
influence  tends  to  act. 

A  unit  magnet  pole  is  one  of  such  strength  that  it  will  repel 
a  like  pole  placed  at  a  distance  of  one  centimeter  from  it  in  air 
with  the  force  of  one  dyne.1 

The  intensity  of  a  magnetic  field  at  a  given  point  is  equal 
numerically  to  the  force  in  dynes  with  which  the  field  acts  on 
unit  pole  placed  at  that  point.  The  unit  of  field  intensity  is 
termed  the  gauss.  Field  intensity  in  air  is  denoted  by  the  letter 
H,  in  other  materials  by  the  letter  B.  The  force  with  which  a 
field  in  air  acts  on  a  pole  of  strength  m  is  expressed  by  the 
formula  F  =  mH.  The  relative  ease  with  which  lines  of  force 
traverse  various  materials  such  as  iron,  nickel,  etc.,  air  being 
the  standard,  is  denoted  by  the  greek  letter  /A,  and  is  called  the 
permeability  of  the  substance  in  question,  so  that 

B  =  Hfi.         For  air  then,  p  =J    /*-  " 


A  field  of  unit  intensity,  or  of  one  gauss,  is  considered  as  hav- 
ing one  line  of  force  per  square  centimeter  of  sectional  area. 

Lines  of  magnetic  force  are  called  maxwells,  and  the  number 
of  maxwells  in  a  field  is  known  as  the  magnetic  flux,  and  is  de- 
noted by  the  symbol  <£.  The  number  of  maxwells  per  unit 
area  (of  one  square  centimeter)  is  the  flux  density  (<£)  and  is 
equal  numerically  to  the  intensity  of  field  H  or  B.  Hence 
<£  =  $  X  area  of  cross-section  of  field  in  square  centimeters. 

By  Coulomb's  law  the  force  in  dynes  between  two  magnet  poles 
in  air  is  equal  to  the  product  of  the  number  of  units  in  each  pole 

1  The  dyne  is  such  a  force  as  will  give  to  a  gram  mass  an  acceleration  of  one  centi- 
meter per  second.  It  is  the  so-called  absolute  unit  of  force.  The  acceleration  due  to  the 
force  of  gravity  is  980  times  as  great. 


MATHEMATICAL  PRINCIPLES 


(that  is  the  product  of  the  pole-strengths)  divided  by  the  square 
of  the  distance  between  them,  and  so  for  all  materials, 


T'fL 


It  follows  from  the  preceding  definitions  that  a  unit  mag- 
net pole  must  create  at  the  distance  of  one  centimeter  from  it 
on  all  sides  a  field  of  unit  strength,  or  one  having  one  max- 
well per  square  centimeter  area.  Since  the  area  of  a  sphere  of 
one  centimeter  radius  is  4?r  square  centimeters,  unit  magnet  pole 
must  have  4-n-  lines  of  force  proceeding  from  it.  The  total  mag- 
netic flux  3>  from  a  pole  of  strength  m  then  equals 


Unit  electric  current  may  be  defined  as  follows:  When  unit 
current  flows  in  one  centimeter  of  wire  in  unit  magnetic  field, 
perpendicular  to  the  lines  of  force,  the  force  between  wire  and 
field  and  perpendicular  to  both  is  one  dyne.  [The  mental  picture 
may  be  formed  by  a  reference  to  Fig  3.]  Hence  current  strength 
(denoted  by  *)  is  force  per  unit  length  of  wire  per  unit  field 
intensity,  or 

*==——     and    F  =  i/H. 
/  M 

This  is  the  absolute  unit  of  current.  The  practical  unit,  the 
ampere,  is  yio  as  large  as  this,  so  that  current  strength  in  prac- 
tical units  is 


/H 

The  absolute  unit  of  electro-motive  force  is  the  e.  m.  f.  in- 
duced in  a  conductor  when  it  cuts  a  magnetic  field  at  the  rate 
of  one  line  of  force  per  second.  Being  a  rate  of  cutting  the  flux, 
it  may  be  expressed  thus 

'=  T    orbetter   «  =       -§-• 

where  d®  =  a  small  portion  of  the  flux  cut  and  dt  =  correspond- 
ing small  portion  of  time.  The  negative  sign  signifies  that  the 
e.  m.  f .  sends  a  current  through  the  conductor  in  such  a  direction 
as  to  demagnetize  the  field. 


THE   MOTOR  AND  THE  DYNAMO 


This  unit  is  extremely  small,  it  taking  ios  or  100,000,000  of 
them  to  make  one  volt.    Hence 

E  volts  = 


io8/  ' 


(b)  The  Induced  Current. 

To  find  the  e.  m.  f .  induced  in  a  straight  wire  moving  sidewise 
across  a  uniform  magnetic  field  in  a  direction  perpendicular 
to  the  lines  of  force.  Let  the  dots  in  Fig.  4  represent  lines  of 
force  passing  vertically  through  the  page. 

i 


H 


1  V* 


Fig.  4- 

Consider  the  wire  of  length  /  centimeters  to  be  moved  with 
velocity  v  centimeters  per  second  in  the  direction  of  the  arrow.  In 
field  of  strength  H  (=  <£)  lines  per  square  centimeter  the  whole 
wire  in  time  t  will  move  vt  centimeters  and  describe  an  area  Ivt 
and  cut  Hlvt  lines  of  force  =  <j>lvt  =  <£.  The  rate  of  cutting 


would  be  — - —        —    —  e  absolute  units  of  e.  m.  f . ,  also  e  =  Htv. 

Now  if  there  be  a  current  i  caused  to  flow  in  this  wire  by 
the  induced  e.  m.  f.,  a  force  F,  will  act  in  a  direction  opposite 
to  the  arrow,  tending  to  prevent  the'  motion  of  the  wire.  The 
rate  of  overcoming  this  force  will  be  Fz/  ergs1  per  second. 

1  An  erg  is  the  absolute  unit  of  work,  being  the  work  accomplished  by  a  dyne  acting 
through  the  distance  of  one  centimeter. 


MATHEMATICAL   PRINCIPLES  7 

But  ei  also  expresses  this  rate  of  working,  current  multiplied 
by  e.  m.  f .  being  power.  Hence 

F^  =  ei  =  liHv,     whence    F  =  UH 

as  before.  Now  /H  is  the  flux  cut  in  each  centimeter  of  the 
motion,  and  if  we  let  d  be  the  number  of  centimeters  moved 
over,  the  total  flux  cut  will  be  IHd  =  $  and  Fd  =  i&. 

But  force  times  distance  is  work;  hence  the  work  of  moving 
the  wire  so  as  to  induce  in  it  a  current  i  is  i®  ergs.  Conversely, 
the  work  done  by  a  flux  <£  in  causing  a  wire,  bearing  current  i, 
to  move  so  as  to  cut  all  the  lines  of  3>  is  i®  ergs.  This  follows 
from  Lenz's  law,  the  directions  of  motion  in  the  two  cases  being 
opposite. 

In  practical  units,    ei    ergs  per  second   becomes  102  — g  volt 

amperes  or  watts.  Since  one  watt  is  io7  ergs  per  second,  the 
number  of  units  of  this  denomination  in  any  given  power  must 
be  multiplied  by  io7  to  equal  the  number  of  absolute  units  ex- 
pressing the  same  power. 

The  preceding  discussion  is  again  one  aspect  of  Lenz's  law. 
The  work  of  maintaining  an  induced  current,  except  for  the 
electrical  resistance  of  the  conductor,  consists  in  overcoming 
the  opposing  force  which  the  induced  current  itself  sets  up  in 
conjunction  with  the  field.  From  this  it  is  at  once  evident 
where  the  power  of  the  engine  goes  which  drives  the  dynamo  of 
a  central  station.  The  converse  of  this  proposition  will  be  found 
on  a  later  page  in  the  more  detailed  discussion  of  the  motor. 

(c)  Induced  Magnetic  Flux. 

It  remains  for  the  present  chapter  to  investigate  the  formation 
of  the  magnetic  flux  by  the  field  coils  of  the  dynamo  machine. 

Consider  the  magnetic  field  inside  a  long  solenoid,  or  better,  in 
order  to  avoid  the  problem  of  the  ends,  consider  the  field  within 
a  toroid. 

Suppose  the  field  to  be  uniform,  and  let  the  number  of  turns 
of  wire  be  denoted  by  N.  If  unit  pole  be  carried  once  around 
the  circular  axis  of  the  toroid,  its  4?r  lines  of  magnetic  force  will 


8 


THE)  MOTOR  AND  THE  DYNAMO 


cut  N  turns,  inducing  in  them  a  current  i.  We  have  seen  that 
the  work  done,  when  a  conductor  carrying  a  current  i  cuts  <£  lines 
of  force,  is  i®  ergs.  Hence,  considering  the  electrical  resistance 
of  the  wire  to  be  zero,  the  total  work  done  in  carrying  unit 
pole  once  around  the  circuit  will  be  47rNj  ergs.  According  to 
Lenz's  law,  this  work  is  done  against  the  opposing  force  of  the 


Solenoid. 


Toroid. 
Fig.  5. 

strength  of  field  within  the  toroid,  due  to  current  i,  namely  H. 
But  H  is  also  the  force  in  dynes  between  unit  pole  and  field  H. 
Hence,  if  /  centimeters  is  the  length  of  the  circular  axis,  the  total 
work  =  force  X  distance  —  HI  —  4^1  ergs. 

This  must  be  the  work  done  by  current  i  in  the  coil  of  N  turns 
to  maintain  a  strength  of  field  H,  and  is  known  as  the  magneto- 
motive force,  M.M.F. 

After  the  analogy  of  Ohm's  law  for  the  electric  current,  we 
have  for  the  magnetic  circuit  the  formula 

M.M.F. 
Flux  =  $  =  — r— 

reluctance 

If  ^  be  the  area  of  the  field,  in  the  present  case  the  area  of  a  loop 


MATHEMATICAL  PRINCIPLES 


9 


of  a  toroid,  then  3>  =  Hs  for  air  and  <£  =  HJ/A  for  other  sub- 
stances whose  permeability  /*  is  different  from  unity.  Hence 
we  may  write 


or 


or  if  I  =  amperes, 


47TNI 


10 


The  reason   for  putting  the  formula  in  this   form  is  that  the 

numerator  is      M.M.F.  and  the  denominator  --  is  the  reluctance 

SIM 

of  the  magnetic  circuit.  This  reluctance,  like  electrical  resistance, 
varies  directly  as  the  length  (/),  inversely  as  the  cross  section 
(j),  and  inversely  as  the  permeability  (/A),  which  last  is  in  a  way 
similar  to  the  conductivity  of  an  electric  conductive  material. 

Knowing  these  last  mentioned  quantities  of  a  magnetic  circuit, 
as  for  instance  of  a  dynamo  machine,  it  is  possible  to  compute 
the  number  of  ampere  turns  (NI)  required  to  produce  a  given 
flux 


(d)  Magnetization  Curves. 

Suppose  a  piece  of  iron,  such  as  the  ring  of  the  toroid  already 

CO 


H' 


S' 


Fig.  6. — B  and  H  curve. — Hysteresis. 


used,  to  be  initially  unmagnetized,  and  imagine  a  constantly 
increasing  magnetizing  force  to  be  applied  to  it,  such  as  the 
current  in  the  winding.  If  the  varying  values  of  H  represent 


10  THE:  MOTOR  AND  THE  DYNAMO 

the  intensity  of  field  that  would  be  produced  in  air  by  this  increas- 
ing M.M.F.  and  B  represent  at  each  instant  the  field  intensity 
produced  in  the  iron  core,  then  a  curve  plotted  between  H  and  B 
would  have  the  form  O^  of  Fig.  6. 

Since  B  =  /*H,  and  the  curve  is  not  a  straight  line,  the  per- 
meability of  the  iron,  /x,  must  be  a  varying  quantity.  Unlike  its 
electric  opposite,  resistivity,  or  specific  resistance,  it  changes  with 
the  flux-density  of  the  iron.  At  the  point  s,  when  H  has  the 
value  H',  the  increment  of  H  is  no  greater  than  the  increment 
of  B,  that  is,  the  iron  has  no  longer  any  multiplying  effect  on 
the  flux  and  has  reached  its  saturation  point. 

If  now  the  magnetizing  current  be  gradually  decreased  to 
zero,  reversed  and  increased  again  in  the  reverse  direction,  the 
B  and  H  curve  will  return  from  s  along  line  scs'.  If  the  current 
be  again  decreased,  reversed  and  increased  to  its  maximum  value 
the  curve  s'c's  will  result.  The  open  space  between  these  curves 
represents  <the  difference  between  the  work  done  in  producing  a 
flux  in  the  iron  and  the  returned  energy  furnished  by  the  dying 
out  of  part  of  this  flux.  In  order  to  demagnetize  the  iron  com- 
pletely, the  current  must  be  reversed,  producing  a  value  of  H 
equal  to  the  distance  from  o  to  where  the  curve  cuts  the  H  axis. 
This  is  known  as  the  coercive  force  and  results  from  the  retentiv- 
ity  for  magnetism  possessed  more  or  less  by  all  forms  of  iron  at 
ordinary  temperatures,  but  in  the  largest  degree  by  hard  steel.  In 
the  complete  cycle  represented,  a  portion  of  the  M.M.F.  goes  to 
overcoming  this  retentivity;  and  in  a  succession  of  reversals,  as 
in  the  case  of  an  alternating  current,  the  loss  of  energy  results  in 
heating  the  iron.  Its  value  in  ergs  is  represented  by  the  area  en- 
closed by  the  curve  and  is  known  as  hysteresis. 

The  empirical  hysteresis  formula  of  Steinmetz  for  various 
sorts  of  iron  is  w  =  ^B1'6,  where  w  is  the  loss  an.  ergs  per  unit 
volume  of  iron  per  magnetic  cycle,  B  is  the  maximum  value 
obtained  by  induction  during  the  cycle,  and  y  is  a  constant  depend- 
ing on  the  quality  of  iron  used. 

(e)  The  Flow  of  Current. 

Ohm's  law  states  that  current  strength  is  proportional  directly 


MATHEMATICAL  PRINCIPLES  II 

to  the  e.  m.  f  .  and  inversely  to  the  resistance.  In  practical  units 
this  is 

volts 

Amperes  —   —  r—  -  . 
ohms 

Whenever  current  flows  through  a  conductor  overcoming  the 
resistance,  it  is  at  the  expense  of  pressure,  or  there  is  always 
present  an  IR  drop.  This  is  a  fundamental  law  of  all  electrical 
circuits.  Without  this  IR  drop  in  voltage  no  current  can  flow. 

Since  IR  =  E  and  IE  =  power,  substituting,  PR  =  the  power 
lost  when  a  current  I  flows  through  resistance  R  by  virtue  of 
pressure  E.  If  the  denominations  are  amperes  and  ohms,  then 
PR  =  watts.  These  lost  or  consumed  watts  go  to  heating  the 
conductor,  and  the  number  of  calories  of  heat  developed  is  0.24 
I2R/  ,  where  t  is  the  time  in  seconds. 

The  resistance  of  a  conductor  varies  directly  with  the  length 
and  inversely  as  the  cross-section.  It  also  varies  with  the 
material  and  with  temperature.  The  specific  resistance  of  a  con- 
ducting material  is  the  resistance  per  mil  foot.  A  mil  foot  is  a 
wire  one  foot  long  and  a  circular  mil  in  cross-section.  A  cir- 
cular mil  is  the  area  of  a  circle  1/1000  mcn  m  diameter.  The 
specific  resistance  for  hard  drawn  copper  wire  at  o°  C.  is  9.7 
ohms. 

As  to  temperature  the  formula  states  that 

R^o  =  R0o  (  i  -f  at}  , 

where  a  is  known  as  the  temperature  coefficient.  For  copper 
a  =  0.0042  ;  for  manganin,  an  alloy  of  manganese,  a  =  almost  o. 
For  carbon  a=  minus  0.0004  (about).  At  20°  C.,  therefore, 
the  resistance  in  ohms  of  a  copper  wire  is 


circ.  mils 


CHAPTER  III. 


THE  DYANMO  MACHINE. 

This  chapter  is  devoted  to  a  general  description  of  the  direct- 


Fig.  7.— The  D.  C.  generator. 

current  dynamo  machine  and  to  a  detailed  explanation  of  its 


THE  DYNAMO   MACHINE  13 

various  organs,  their  functions,  and  the  materials  used  in  their 
construction. 

Fig.  7  represents  the  general  arrangement  of  the  parts  of  the 
most  common  type  of  machine.  It  is  a  two-pole  shunt-wound 
dynamo,  and  may  be  used  either  as  motor  or  generator 
indifferently.  We  will  consider  it  as  a  generator. 

By  the  term  shunt-wound  is  signified  the  method  of  exciting 
the  field  coils,  the  current  for  them  being  tapped  from  the 
machine  terminals  T  T'  by  a  circuit  which  forms  a  shunt,  or 
by-path  sh,  to  the  main  line-circuit  L  I/.  The  armature  of  this 
machine  is  drum-wound,  its  wires,  or  inductors,  being  on  the 
surface  of  a  rotating  iron  cylinder  which  forms  the  armature- 
core.  These  wires  are  carried  in  sixteen  slots  on  the  lateral  sur- 
face of  the  core  and  form  by  themselves  a  closed  circuit.  They 
are  connected  at  regular  intervals  to  the  eight  bars  of  the  com- 
mutator C.  The  current  is  taken  off  from  the  commutator  by 
the  brushes  BB'. 

By  applying  Ampere's  rule  for  direction,  it  will  be  seen  that 
in  those  inductors  whose  section  is  represented  thus  • ,  the  cur- 
rent is  flowing  toward  the  observer;  and  in  those  represented 
thus  o,  it  is  flowing  from  the  observer. 

Though  the  diagram  shows  only  one  inductor  to  a  slot,  it  is  to 
be  understood  that  there  are  usually  several  wires  in  each  slot, 
the  windings  of  two  connected  slots,  as  15  and  8  or  4  and  n, 
being  repeated  a  definite  number  of  times  before  proceeding  to 
the  next,  such  a  group  constituting  an  armature-coil  or  winding- 
element. 

Starting  with  the  commutator  bars  touching  the  upper  brush 
and  attached  to  inductor  (or  winding-element)  No.  10  or  I  it 
will  be  seen  that  we  may  proceed  by  two  paths  and  finally  arrive 
at  a  bar  which  touches  the  opposite  brush,  namely,  by  numbers 
12,  3,  14,  5,  16,  7,  also  2  and  9,  or  by  numbers  15,  8,  13,  6,  n, 
4,  also  9  and  2. 

Proceeding  thus  from  the  negative  to  the  positive  brush,  the 
e.  m.  f.  generated  in  each  inductor  or  winding-element  is  added 
to  that  generated  in  the  next  one  connected  with  it,  so  that  the 


14  THE)  MOTOR  AND  TH£  DYNAMO 

effect  is  the  same  as  that  of  two  series  chains  of  voltaic  cells,  the 
two  chains  being  in  parallel  from  brush  to  brush. 

Although  in  any  one  inductor  or  winding-element  the  current 
is  an  alternating  one,  changing  direction  twice  in  each  revolution 
of  this  bi-polar  machine,  yet  the  inductors  to  the  left  of  the  axis 
of  commutation  ax  always  generate  an  e.  m.  f.  in  one  direction 
and  those  to  the  right  of  ax  in  the  other  direction.  The  rotating 
armature  is  thus  a  sort  of  double  electric  pump,  in  which  the  two 
cylinders  work  in  unison  converting  mechanical  energy  into  elec- 
trical energy.  In  a  machine  of  100  per  cent,  efficiency  this  con- 
version of  energy  would  be  perfect  according  to  Lenz's  law. 

The  elementary  alternating-current  dynamo  does  not  differ 
from  the  machine  just  described  except  in  two  particulars. 
First,  the  commutator  is  replaced  by  two  insulated  rings  con- 
nected each  to  diametrically  opposite  armature  inductors,  as 
8  and  16,  or  4  and  12,  etc.,  all  other  inductors  being  connected  as 
in  the  diagram,  but  not  directly  to  the  rings.  The  other  differ- 
ence is  that  the  field  is  excited  by  a  different  current,  either  from 
some  outside  source  or  by  aid  of  a  special  commutator  for  the 
field  current  alone.  Alternating-current  machinery  is  treated  in 
later  chapters. 

We  are  now  in  a  position  to  write  the  general  formula  for 
the  e.  m.  f.  generator  in  a  direct-current  dynamo  in  accordance 
with  principles  laid  down  in  Chapter  II.  Let 

$  =  the  total  flux  passing  from  any  one  pole  to  the  neighbor- 

ing part  of  the  armature  core,  or  vice  versa. 
p  :=  the  number  of  poles. 

N  =  the  total  number  of  inductors  on  the  lateral  surface  of 
the  armature,  that  is,  in  the  slots. 

p'  =  the  number  of  parallel  paths  through  the  armature  wind- 
ings from  the  —  to  the  -f-  terminal  of  the  machine. 

n  =  the  revolutions  per  second. 


Then  the  generated  e.  m.  f.  in  volts  —  E  =  -^-§Tr~  »    because 

gen.      IO/> 

the  total  flux  cut  by  the  armature  windings  is  &p,  and  the  flux 


*k         THE  DYNAMO  MACHINE  15 

cut  each  second  by  one  wire  on  the  lateral  surface  of  the  arma- 

N 
ture  is  $pn.     The  number  of  such  wires  in  series  is  — r  . 

Now  this  amount  of  e.  m.  f.  is  generated  whether  the  machine 
be  turned  by  some  outside  agent,  such  as  a  steam  engine,  or 
whether  it  rotate  as  a  motor  through  the  agency  of  current  fed 
into  its  armature,  because  of  the  field  flux.  In  the  latter  case, 
in  order  to  feed  this  current  into  the  armature,  therefore,  a 
slightly  higher  e.  m.  f.  is  required  than  that  generated,  and  in  the 
reverse  direction,  sufficient  indeed  to  overcome  the  resistance  of 
the  armature  windings,  brushes,  brush  contacts,  etc.,  which  we 
may  denote  as  Rrt.  This  additional  e.  m.  f.  may  be  designated 
Ia  Rrt,  where  Ia  is  the  armature  current.  In  this  instance, 
namely  that  of  the  motor,  the  generated  e.  m.  f.  is  termed  a 
counter  electro-motive  force,  being  opposed  in  direction  to  the 
e.  m.  f.  of  the  line  which  supplies  the  driving  power. 

Similarly  the  e.  m.  f.  at  the  terminals  of  a  dynamo  never  is 
as  high  as  the  generated  e.  m.  f.,  when  the  dynamo  is  furnishing 
current,  because  a  part  of  this  generated  e.  m.  f.  is  used  up  in 
forcing  such  load-current  through  the  resistance  of  the  armature, 
etc.,  R,. 

Thus  the  formulae  for  the  shunt  dynamo  furnishing  current 
and  for  the  motor  are  as  follows :  For  the  former  the  terminal 
voltage  E,  =  E,  —  Ia  R,  or 


o 
and  for  the  shunt  motor  the  line   voltage   E/  =  E^  -}-  Irt  Rrt  or 


In  order  to  make  the  formulae  general  for  all  direct  cur- 
rent machines,  it  becomes  necessary  to  notice  here  the  two  other 
ways  used  to  excite  the  field.  Instead  of  letting  the  field  circuit 
form  a  shunt  or  by-path,  the  full  armature  current  may  be  di- 
rected through  the  field  windings,  forming  a  series  machine,  or 
a  combination  of  these  two  methods  may  be  used,  as  in  the 
compound  machine.  In  the  last  case  the  shunt  winding  of  the 


i6 


THE)  MOTOR  AND  THE  DYNAMO 


field  is  augmented  by  a  few  turns  in  series  with  the  main  circuit. 
Fig.  8  represents  the  three  usual  methods  of  field  excitation. 


Shunt. 


Series. 


Compound. 
Fig.  8. 

Letting  R^  represent  the  resistance  of  the  series  field  windings, 
the  preceding  formulae  appear  as  follows  for  bath  series  and 
compound  machines. 

For  the  dynamo    E,  =  E,  —  I*  Rrt  —  I«  R,. 
For  the  motor        E,  =  E,  +  I,  R,  -f  L  R,. 

The  parts,  or  organs,  of  the  direct-current  dynamo  to  be  treated 


THE  DYNAMO   MACHINE 


of  in  detail  are  as   follows :    frame  and  field  cores,   armature 
cores,  field  windings,  armature  windings,  commutator,  brushes, 


Fig.  9.— General  Electric  Co.  5<>-kilowatt  generator.         / 

brush  holders,  bearings,  lubricating  devices,  insulating  materials. 

(a)  Frame  and  Field  Cores. 

The  various  shapes  of  dynamo  frames  are  shown  in.  Figs.  10, 
n,  12  and  13.  The  bipolar  type  is  seldom  made  larger  than  5 
kilowatts.  The  particular  advantages  of  the  multipolar  type  may 
be  enumerated  as  follows: 

i.  The  length  of  the  magnetic  circuit  is  shorter  in  the  multi- 
polar  type,  thus  making  the  machine  more  compact  and  reducing 
the  weight. 


i8 


THE  MOTOR  AND  THE  DYNAMO 


2.  The  lower  reluctance  of  the  multipolar  machine  reduces  the 
ampere  turns  required  for  excitation,  which  results  in  a  saving  of 
copper. 

3.  The  number  of  revolutions  per  minute  is  reduced  by  in- 


Fig.  io.— Bell  Motor  Co.    Early  model.    Open  form. 


Fig.  ii. — Crocker-Wheeler  Co.    Enclosed  form,  now  used  by  many  manufacturers. 

creasing  the  number  of  poles,  thus  rendering  the  generator  bet- 
ter adapted  for  direct  coupling  to  a  reciprocating  engine.  Fur- 
thermore with  the  same  peripheral  speed  the  larger  armature 
renders  the  centrifugal  force  less.  For  instance,  if  the  number 


THE  DYNAMO   MACHINE  IQ 

of  poles  be  made  say  four  instead  of  two,  the  pole-face  not  be- 
ing reduced  in  size,  in  order  to  have  the  same  number  of  in- 
ductors under  one  pole,  the  armature  will  have  to  be  doubled  in 
circumference.  But  with  the  same  peripheral  speed,  the  revolu- 
tions per  minute  would  be  only  half  as  great ;  and  the  centrifugal 

force,  which  is  —     -  ,  would  also  be  reduced  to  half.     This  ideal 

advantage,  however,  does  not  occur  in  practice,  the  gap  between 
the  poles  being  necessarily  larger  in  a  multipolar  machine  than  in 


Fig.   12.  -Enclosed  type.    Four-pole  frame. 

a  bipolar,  in  order  to  avoid  magnetic  leakage.  In  direct-current 
machines  it  is  the  custom  to  let  the  pole-faces  span  from  60  per 
cent,  to  75  per  cent,  of  the  total  armature  circumference.  The 
frame  is  usually  cast  in  sections  and  bolted  together.  The  pole 
cores  are  sometimes  made  of  laminated  iron,  and  in  some  machines 
they  are  capped  by  a  projecting  plate  or  "shoe." 


2O  THE  MOTOR  AND  THE)  DYNAMO 

The  material  of  frame  and  cores  is  chosen  with  reference  to 
its  magnetic  properties.  The  B  and  H  curve  mentioned  in  chap- 
ter II  for  various  sorts  of  iron  is  shown  in  Fig.  14. 

Cast  iron,  although  the  cheapest  of  the  varieties  mentioned, 
requires  more  ampere  turns  than  any  other  to  magnetize  it  to 
the  proper  flux-density.  Hard  steel  is  never  employed  for  any 


Fig.    13. — Complete  field  frame  of  600  k.w.  generator.     G.  E.  Co. 

but  permanent  magnets,  such  as  are  used  in  magnetos  and 
electrical  measuring  instruments.  Its  hysteretic  qualities  render 
it  especially  valuable  for  such  purposes.  The  best  material  for 
field  cores  is  mild  steel,  and  this  is  often  employed  for  the  frame 
as  well.  The  base  of  the  machine,  however,  and  remote  parts  of 
the  magnetic  circuit  are  not  infrequently  made  of  cast  iron. 


THE  DYNAMO   MACHINE 


21 


(b)  Field  Windings. 

The  shunt  field  windings  of  a  direct-current  machine  are  de- 
signed to  carry  from  about  2  per  cent,  to  8  per  cent,  of  the  full 
load  current  of  the  machine.  The  number  of  ampere  turns  re- 


io      30      to     So      to      70      »o     90     »oa    UQ 


AOOQ 


Fig.  14. 

quired  for  field  excitation  can  be  determined  from  a  formula  of 
Chapter  II,  namely. 

47rNI 


or       NI  = 


.  10  - 


47 


The  magnetic  circuit  is  made   up  of  various  parts,   each  of 
which  must  be  separately  computed,  owing  to  the  different  values 


22  THE  MOTOR  AND  THE  DYNAMO 

of  flux  density  commonly  employed  in  each.  For  instance  let 
the  field  cores  be  9f  soft  cast  steel  and  their  dimensions  be  as 
follows:  /  =  50  centimeters,  j  =  J>525  square  centimeters, 

4> 

<$  =  23,000,000  maxwells.  Hence  B  numerically  =  -  is  ap- 
proximately 15,000  units.  Consulting  Fig.  14  it  will  be  seen  that 

T> 

for  soft  cast  steel,  when  B  =  15,000,  H  =  35  and  hence  /*  =  -  ~ 

rl 

10  X  15,000  X  — 

=  430  (about).     Thus  NI  becomes  -  —  =  1430 

4*" 

ampere  turns. 

For  the  other  parts  of  the  circuit  such  as  the  field-yoke  or 
frame,  the  armature  core  and  teeth,  and  the  air-gap,  for  which 
the  permeability  is  one,  individual  computations  have  to  be  made. 
The  sum  of  the  various  ampere  turns  found  is  then  the  total  num- 
ber required,  for  each  pair  of  poles.  Dividing  by  2  gives  the 
ampere  turns  for  one  field  coil  of  the  series  or  the  shunt  machine. 
Magnetic  leakage  has  not  been  considered  in  this  calculation : 
see  page  48. 

The  size  of  wire  used  and  the  dimensions  of  the  coil  de- 
pend on  the  allowable  rise  of  temperature.  The  formula  for 
heat  developed  by  an  electric  current,  given  in  chapter  II,  is  H 
in  calories  =  0.24  I2R/,  where  R  is  the  resistance  of  the  wire  in 
ohms.  The  allowable  temperature  rise  is  50°  C.  above  the  ordi- 
nary machine-room  temperature  of  25°  C.,  or  a  maximum  tem- 
perature of  75°.  When  this  point  is  reached,  the  exposed  surface 
of  the  coil  should  be  sufficient  to  radiate  in  the  cooling  breeze 
furnished  by  the  rotating  armature  all  heat  energy  which  would 
tend  to  elevate  the  temperature  above  75°.  Manufacturers  have 
various  empirical  formulae  for  approximating  the  relative  dimen- 
sions of  the  spool. 

The  winding  of  the  field  may  be  round  copper  wire  or  in  the 
form  of  ribbon.  The  insulation  is  usually  of  cotton  and  occupies 
from  30  per  cent,  to  60  per  cent,  of  the  total  cross  section  of 
the  winding.  The  coils  are  usually  held  in  shape  by  paper  or  cord, 


THE  DYNAMO   MACHINE  23 

and  the  surface  is  covered  with  moisture-proof  varnish.     See 


(c)  Armature  Core. 

The  armature  core  is  made  of  iron  or  steel,  being  a  very  con- 
siderable part  of  the  magnetic  circuit.  Since  the  core  cuts  lines 
of  magnetic  force  as  well  as  the  armature  windings,  in  a  solid 
core  there  would  be  nothing  but  the  resistance  of  the  iron  to 


Fig.  15.— G.  E.  Co.— Field-coil  of  B.C.  generator,  200  K.W.  and  above. 


Fig.  16.— Edgewise  field  winding.    Crocker-Wheeler  Co. 

prevent  a  heavy  induced  current  from  flowing  longitudinally 
down  one  side  of  the  core  and  back  on  the  other  side,  wasting 
the  energy  of  the  machine  and  heating  the  iron.  Such  currents 
do  in  a  measure  occur  and  are  known  as  eddy,  or  Foucault,  cur- 
rents. They  are,  however,  to  a  large  extent  prevented  by  build- 
ing up  the  core  of  sheet  iron,  the  laminations  being  at  right  angles 
to  the  shaft.  Their  thickness  in  higher  priced  machines  varies 
3 


24  THK  MOTOR  AND  THE  DYNAMO 

from  0.014  to  0.02  inch.  For  insulating  electrically  the  lamina- 
tions from  one  another  some  manufacturers  use  varnish,  others 
simply  depend  on  the  oxidation  of  the  iron  surface.  Because  of 
the  continual  reversal  of  the  magnetic  flux  through  the  rotating 
core,  a  sheet  iron  or  steel  with  a  low  hysteretic  constant  y  is 
desirable. 

Two  general  shapes  of  armature  core  prevail  in  direct-current 
machines,  the  drum  and  the  ring.  In  the  original  machines  built 
and  operated  by  Gramme  at  Vienna  the  ring  armature  was  used. 
The  disadvantage  of  this  type  is  that  wires  on  the  outer  sur- 


Fig.  17. — Gramme  ring. 

face  only  cut  the  field  flux,  the  return  wires  through  the  ring 
serving  merely  as  connectors.  See  Fig.  17.  The  advantage  of 
this  type  of  armature  core  is  the  superior  ventilation  and  cooling. 
In  all  but  the  smallest  machines,  armature  cores  are  now  cooled 
by  radial  ducts,  and  the  drum-wound  core  is  the  prevailing  form. 
The  ring-shaped  core  is  sometimes  used  wound  as  a  drum,  the 
laminated  portion  being  supported  on  a  star-shaped  cast-iron 
frame  secured  to  the  shaft,  but  in  small  machines  the  laminations 
for  the  drum-winding  set  directly  on  the  shaft.  In  the  earlier 
machines  the  windings  were  secured  to  the  core  surface  by 


THE;  DYNAMO  MACHINE;  25 

wooden  pins.     The  modern  method  is  to  insert  the  inductors 
into  lateral  grooves  or  slots  on  the  core  surface. 

Fig.  1 8  shows  a  number  of  shapes  of  these  armature  laminae 
or  punchings.  The  parts  between  the  grooves  are  known  as 
the  armature  teeth.  As  the  machine  rotates,  the  field  flux  sweeps 
across  in  tufts  from  tooth  to  tooth  cutting  through  the  inductors 
in  the  slots.  The  narrowest  possible  air  space  between  armature 


Fig.  18. 

surface  and  field  magnet  face  is  therefore  not  always  the  most 
effective. 

(d)  Armature  Windings. 

The  armature  inductors  must  be  of  sufficient  thickness  to  carry 
the  full  load  current  of  the  machine  without  undue  heating.  They 
are  usually  of  circular  cross  section  and  cotton  insulated,  but 
on  larger  machines  ^and  particularly  in  alternating-current  gen- 
erators they  are  in  the  form  of  ribbon.  They  are  usually  wound 
on  forms  independently  of  the  armature  core,  a  number  of  turns 
of  wire  constituting  a  winding-element.  These  elements  are 
then  placed  into  position  in  the  armature  slots  and  properly 
secured  and  taped.  The  ends  are  finally  connected  to  the  com- 
mutator bars  according  to  the  design  of  the  winding.  The  slots 
are  sometimes  lined  with  strips  of  paper  or  insulating  fiber 
before  the  introduction  of  the  wires,  and  in  certain  cases  the 
inductors  are  held  in  position  behind  a  strip  of  insulating  ma- 
terial that  fits  in  a  groove  in  either  tooth,  thus  covering  the  slot. 
See  Fig  18.  The  even  distribution  of  the  windings  within 
the  slots  is  a  matter  of  great  importance,  especially  in  high 
speed  machines,  as  any  inequality  renders  the  armature  poorly 


26 


THE:  MOTOR  AND  THE:  DYNAMO 


balanced  and  causes  jarring.  When  all  the  slots  are  filled,  the 
inductors  are  secured  firmly  against  displacement  from  centrifugal 
force  by  means  of  circular  bands  of  brass  or  even  steel  wire.  Figs. 
19,  20,  21,  and  22  show  various  types  of  armatures. 


Fig.  19.— Complete  armature.    Fort  Wayne  Electric  Co. 


Fig.  20. — Armature  in  process  of  winding.    G.  E.  Co. 

The  succession  of  inductors  on  the  armature  surface,  that  is, 
the  methods  of  connecting  the  winding  elements  into  a  system, 
are  various;  but  in  the  closed  coil  type  two  chief  forms  of  arm- 


THE  DYNAMO  MACHINE  2.J 

ature  winding  prevail.     They  are  the  lap  winding  and  the  wave 


Fig.  21. — Complete  A.  C.  armature.    G.  E.  Co. 


Fig.  22. -IyO\v  speed  armature.    150  r.p.m.    Crocker-Wheeler  Co. 

winding.     The  open  circuit  type,  namely  that  in  which  the  ar- 


28  THE;  MOTOR  AND  THE;  DYNAMO 

mature  windings  do  not  in  themselves  make  a  closed  circuit,  oc- 
curs only  in  arc-lighting  generators,  and  must  be  treated  later. 
The  armature  depicted  in  Fig.  7  is  of  the  drum  type.  It  will  be 
observed  that  in  proceeding  around  such  an  armature  the  al- 
ternate slots  only  are  used,  the  intermediate  ones  being  left  free 
for  the  return  wires  of  some  other  winding  element.  Thus  12 
and  3  are  followed,  not  by  13  and  4,  but  14  and  5.  The  neces- 
sity for  this  arrangement  becomes  at  once  apparent  from  Fig.  23, 
which  represents  an  attempt  to  wind  an  armature  using  successive 
slots.  It  will  be  observed  that  one  must  pass  twice  around  the 
core  to  form  a  closed  coil  winding.  This  can  be  easily  proven 
by  trying  it  with  a  ball  of  string. 


Fig.  23. 


The  drum  armature  is  a  development  of  the  ring  type,  the 
return  wires  through  the  center  of  the  ring  being  carried  in- 
stead to  the  opposite  side.  In  the  ring  wound  armature,  every 
inductor  or  every  second  or  third,  etc.,  inductor  may  be  con- 
nected to  a  comna£nl^itor  bar,  making  the  maximum  number  of 
bars  equal  to  the  number  of  inductors  or  groups  of  inductors.  In 
the  drum  type,  on  the  other  hand,  there  can  be  only  one  bar  for 
every  two  inductors  or  bundles. 

The  essential  differences  between  the  lap  and  the  wave  wind- 
ing for  armatures  may  be  best  appreciated  by  a  study  of  Fig.  24. 
The  brushes  are  diagramed  on  the  inside  of  the  commutator  so 
as  not  to  obscure  the  windings. 


THE  DYNAMO   MACHINE; 


I,ap  wniding. 


Wave  winding. 
Fig.  24. 


3O  THE  MOTOR  AND  THE  DYNAMO 

General  observations  applicable  to  both  windings : — 

(a)  There  are  half   as  many  commutator  bars  as   there   are 
inductors  (or  bundles  of  conductors)  on  the  armature  surface. 

(b)  The   space    or   "pitch"    between   two    directly   connected 
inductors   or   bundles,   which   thus    form   a   winding-element,   is 
approximately  such  that  when  the  one  is  entering  the  flux  from 
a  north  pole,  the  other  is  entering  the  flux  from  the  south  pole, 
etc.    This  causes  oppositely  directed  e.  m.  f .  to  be  induced  in  the 
two  sides  of  the  winding-element.     It  follows  that  the  distance 
between  two  such   inductors   must  be   neither   so  small   as   the 
width  of  a  field  pole  face  nor  so  large  as  to  reach  much  beyond 
corresponding  points  of  two  consecutive  pole-faces.     Otherwise, 
the  e.  m.  f .  induced  in  the  two  sides  of  a  winding  would  be  in 
the  same  direction. 

(c)  In  the  simplex  winding  the  spacing  or  pitch  must  always 
be  an  odd  number,  otherwise  all  the  slots  will  not  be  filled. 

(d)  It   would  be   perfectly   possible   to  have   either  half   the 
number  of  slots  in  the  armature  surface,  there  being  two  wind- 
ing-elements to  a  slot,  or  twice  the  number  of  slots,  there  being 
then  half  a  winding-element  in  each  slot. 

(e)  It  would  be  possible  to  sandwich  in  between  the  slots  and 
bars  represented  a  second  set  of  slots  and  bars  equal  in  number 
and  carrying  a  second  and  independent  winding.     This  would  be 
known   as   a   duplex  winding   and   the   brushes   would   be   wide 
enough  to   cover  two  commutator   bars   instead   of   one.     In   a 
similar  way  a  triplex  winding  could  be  formed,  the  brushes  then 
covering  three   consecutive   commutator   bars.      By  this   means 
each  winding  would  be  made  to  carry  only  a  half  or  a  third  of 
the  entire  current.     Such  types  of  winding  as  close  upon  them- 
selves forming  a  single  continuous  circuit  are  said  to  be  singly 
re-entrant.     All   simplex   windings   are   so.     A   duplex   winding 
such  as  described,  on  the  other  hand,  would  be  doubly  re-entrant. 
It  could  be  made  singly  re-entrant  by  connecting  the  two  dis- 
tinct windings  in  series. 

(f)  The  commutator's  position  on  the  shaft  with   reference 
to  the  windings  is  immaterial  so  long  as  the  bars  and  slots  are  in 


THE  DYNAMO   MACHINE  31 

proper  sequence.  That  is,  the  wires  extending  from  each  com- 
mutator bar  to  the  periphera  of  the  armature  may  be  of  equal 
length,  curving  equally  in  either  direction,  or  the  one  wire  may 
extend  radially  out  to  the  nearest  slot,  the  other  being  longer 
and  curving  around  to  its  proper  slot.  The  appearance  of  the 
end  of  the  armature  and  the  position  of  the  brushes  with  respect 
to  the  field  poles  will  be  different  in  the  two  cases. 

(g)  The  winding  may  be  either  "right-hand"  ("progressive") 
or  "left-hand"  ("retrogressive"),  according  as  we  proceed  around 
the  armature  clockwise  or  counter-clockwise. 

(h)  The  number  of  slots  chosen  (for  example  22)  is  not 
exactly  divisible  by  the  number  of  poles  (4),  in  order  that  there 
may  be  no  synchronous  vibration  in  e.  m.  f.,  as  might  be  the 
case  if  at  every  instant  each  of  the  four  groups  of  inductors  had 
exactly  the  same  position  with  reference  to  each  of  the  four 
poles.  ,,v 

In  reference  to  the  lap-winding  the  following  observations 
may  be  made  : — 

(a)  The  number  of  brushes  is  equal  to  the  number  of  poles. 
It  will  be  seen  by  a  study  of  the  diagram  that  this  number  is 
necessary  in  order  that  the  same  voltage  may  be  developed  in 
each  armature  path.    In  the  special  case  of  the  figure,  if  we  pass 
from  a  commutator  bar  in  connection  with  a  brush  to  a  brush  of 
the  opposite  polarity,  either  4  or  6  inductors,  that  is,  either  2  or 
3  winding-elements,  connected  in  series,  are  passed  over. 

(b)  The   number  of   paths   in   parallel   is   then   equal   to   the 
number  of  poles  or  in  simplex  winding  p'  =  p.    The  number  of 
inductors  in  series  is  therefore  approximately  N//>. 

(c)  In  the  diagram  the  forward  pitch,  that  on  the  commutator 
end  of  the  armature,  is  5,  the  backward  pitch,  that  on  the  farther 
end,  is  7.    The  average  pitch  is  therefore  6.    Taking  two  slots  to  a 
winding-element,  as  here,  it  will  be  readily  seen  that  in  the  lap- 
winding,  any  even  number  of  slots  may  be  used.    In  a  duplex  or 
triplex  winding,  2  times  or  3  times  such  even  number  respectively 
may  be  used. 


32  THE  MOTOR  AND  THE)  DYNAMO 

In  reference  to  the  wave-winding,  the  following  corresponding 
observations  may  be  made : — 

(a)  Only  two  brushes  are  necessary.     In  the  figure,   if   we 
pass  from  a  commutator  bar  in  connection  with  one  brush  to  the 
brush  of  opposite  polarity,  we  must  pass  over  10  or  12  inductors, 
that  is,  5  or  6  winding-elements,  essentially  double  the  number 
of  the  lap-winding.     To  be  sure,   four  brushes  could  be  used, 
connected  as  in  the  lap-winding.     But  since  the  opposite  side  of 
the  commutator  is  already  connected  with  each  brush  through  an 
armature  winding,  the  only  advantage  of  the  extra  brushes  would 
be  to  divide  the  current  passing  through  each  brush,  which  would 
have  a  tendency  to  reduce  sparking. 

(b)  The  number  of  paths  in  parallel  through  the  armature  is 
only  2  in  a  simplex  winding,  or  p'  —2.      The  number  of  induc- 
tors in  series  is  therefore  N/2. 

(c)  In  the  diagram,  the  pitch  is  5,  the   forward  and  back- 
ward pitches  being  alike,  although  this  is  not  always  necessarily 
the  case.     Taking  two  slots  to  a  winding-element,  it  will  be  evi- 
dent that  the  number  of  slots  must  not  be  evenly  divisible  by 
the  number  of  poles,  otherwise  the  winding  would  close  in  pro- 
ceeding only  once  around  the  armature.    It  must,  on  the  contrary, 
lap  over  one  commutator  bar  or  fall  short  by  one  as  here,  thus 
adding  or  subtracting  two  slots  from  an  evenly  divisible  number. 
Hence  the  number  of  slots  =  pitch  X  number  of  poles  =t  2.     In 
a  duplex  or  triplex  winding  2  or  3  times  this  number  would  be 
used. 

A  comparison  of  the  two  forms  of  winding  makes  it  evident 
that  in  anything  above  a  two-pole  machine,  the  e.  m.  f.  will  be 
higher  with  the  wave-winding,  other  conditions  such  as  flux, 
r.  p.  m.,  etc.,  being  the  same.  In  a  four-pole  machine  the  e.  m.  f. 
would  be  essentially  twice  as  much  with  a  wave-wound  as  with  a 
lap-wound  armature.  For  this  reason  the  wave-winding  was 
formerly  called  series  winding  and  the  lap-winding  was  called 
parallel  winding.  Traction  motors  are  usually  wave-wound. 
Fig.  25  shows  another  form  of  diagram.  It  corresponds  to  the 
wave-winding  of  Fig.  24. 


THE  DYNAMO   MACHINE 


33 


Fig.  25.— Straight  diagram  of  armature  winding. 

(e)  The  Cummutator. 

The  commutator  consists  of  wedge-shaped  bars  of  copper 
insulated  from  the  shaft.  These  bars  are  held  in  place  by  a 
retaining  ring  from  which  they  are  also  insulated. 


Fig.  26.— Commutator.    Electro  Dynamic  Co. 

Hot  forged  copper  is  sometimes  used,  but  cold  rolled  bars  are 
preferable,  because  for  one  reason,  they  can  be  shaped  more 
accurately. 

The  insulating  material  between  the  bars  is  mica,  the  amber 
mica  being  preferred.  The  thickness  varies  from  0.02  to  0.06 
inch.  A  material  known  as  miconite,  consisting  of  powdered 
mica,  formed  into  sheets  under  high  pressure,  is  much  employed. 


34  THE  MOTOR  AND  THE  DYNAMO 

Accuracy  in  the  thickness  of  the  insulation  is  thus  more  easily 
secured  than  with  mica  in  the  natural  state. 

When  the  bars  and  insulators  have  been  assembled,  they  are 
forced  tightly  together  either  by  a  ring  and  clamping  screws  or 
by  means  of  hydraulic  pressure.  The  retaining-rings  are  then 
put  in  place. 

The  armature  windings  are  usually  attached  to  the  commutator 
bars  by  soldering  the  ends  into  grooves.  Although  there  are 


44Q85 ^ 


4489  Z 

,44-QQO 


-44Q84 

—44663 

-4467(5 


Fig.  27.— Structure  of  a  commutator.    G.  E.  Co. 

objections  to  this  method,  owing  to  the  high  temperatures  reached 
sometimes  by  commutators,  yet  it  is  more  sure  than  the  employ- 
ment of  clamps  or  screws,  which  are  likely  to  work  loose. 

The  size  of  the  commutator,  number  of  bars,  etc.,  depend 
directly  upon  the  style  of  armature  winding  employed.  In  a 
general  way,  however,  it  may  be  remarked  that  high  speed 
machines  usually  have  armatures  of  comparatively  small  diam- 
eter, high  voltage  machines  may  usually  be  detected  by  the  com- 


THE  DYNAMO   MACHINE  35 

parative  narrowness  of  the  commutator  bars  and  their  large 
number,  and  machines  for  supplying  large  current,  such  as  elec- 
tro-plating dynamos,  usually  have  rather  long  commutators,  to 
permit  several  brushes  to  be  placed  abreast. 

(f)  The  Brushes. 

In  some  of  the  earliest  types  of  dynamos,  steel  brushes  in  the 
form  of  solid  bars  were  employed,  because  of  the  low  friction. 
Later,  brushes  made  of  strip-copper  were  used  and  still  later  of 
copper  gauze  folded  into  several  thicknesses.  These  are  now 
employed  only  on  low  voltage  machines  furnishing  large  current, 
such  as  plating  dynamos.  The  lower  resistance  of  the  copper 
renders  it  better  fitted  for  such  generators  than  carbon. 

On  the  ordinary  direct-current  dynamos  and  motors,  however, 
graphitic  carbon  pressed  and  shaped  into  block  form  is  the  type 
of  brush  universally  employed.  The  chief  advantage  of  this 
material  is  that  its  high  resistance  aids  to  prevent  sparking  (see 
p.  58).  Besides  this  it  keeps  the  commutator  fairly  clean,  does 
not  wear  out  the  copper  very  rapidly,  and  is  sufficiently  soft  to 
be  readily  shaped  to  the  curved  commutator  surface.  These 
brushes  are  usually  copper-plated  where  they  make  contact  with 
the  brush-holders. 

Carbon  brushes  are  sometimes  set  radially  to  the  commutator, 
but  usually  at  a  slight  angle,  even  in  machines  designed  to  operate 
in  either  direction.  It  makes  little  difference  whether  a  machine 
operate  with  or  against  carbon  brushes. 

The  area  of  brush-contact  depends  upon  the  current  to  be 
carried  and  determines  the  size  of  brush  to  be  employed.  The 
approximate  current  density  per  square  inch  of  contact  area  is 
from  50  amperes  in  no  volt  machines  to  30  amperes  in  550  volt 
machines.  In  cases  where  these  figures  would  call  for  a  very 
wide  brush,  several  are  placed  abreast,  thus  insuring  better  con- 
tact and  more  even  wearing  of  the  commutator. 

(g)  Brush  Holders. 

Types  of  brush-holders  are  represented  in  Figs.  28,  29  and  30. 
The  springs  are  adjustable  so  as  to  maintain  the  proper  pressure 


THE   MOTOR  AND  THE  DYNAMO 


of  the  brushes  against  the  commutator.    A  slight  variation  of  the 
pressure  being  often  sufficient  to  correct  the  defect  of  sparking.  It 


Fig.  28. — Brush  and  holder.     Crocker-Wheeler  Co. 


Fig.  29.— Brush  and  holder.     G.  E.  Co. 

will  be  observed  that  a  flexible  conductor  carries  the  current  from 
the  brush  so  that  it  shall  not  pass  through  the  spring,  which  might 
otherwise  become  heated  and  lose  its  elastic  properties. 


THE  DYNAMO   MACHINE)  37 

The  brush-holders  are  connected  to  a  rocker,  enabling  the 
brushes  to  be  shifted  in  position  around  the  commutator.  The 
rocker  consists  of  a  lever  or  a  collar,  which  bears  both  positive 
and  negative  brushes,  and  may  be  rotated  through  a  few  degrees 


Fig.  30.-G.  E.  Co. 

around  the  axis  of  the  commutator.  In  small  machines  it  is 
operated  by  a  handle,  in  larger  ones  by  a  wheel  and  gearing. 
When  in  the  proper  position,  it  may  be  clamped.  In  interpolar 
variable  speed  motors,  when  the  rocker  has  been  adjusted  in  the 
factory,  it  is  fixed  in  position  by  pins. 

(h)  The  Bearings. 
Fig.  31   shows  an  approved  type  of  bearing,  consisting  of  a 


Fig.  31. — Bearing  housing  open,  showing  bearings,  oil  wells,  and  oil  rings. 
Westinghouse  Co. 

brass  or  bronze  collar  set  in  Babbitt  metal  in  the  iron  support. 


38  THE:  MOTOR  AND  THE;  DYNAMO 

The  figure  also  shows  the  oil-rings  used  for  lubricating.  They 
rest  upon  the  shaft,  and  the  oil  is  contained  in  wells  through 
which  they  rotate. 

Recently  ball-bearings  have  been  employed  in  small  machines 
with  great  success.  They  require  little  attention  and  the  lubri- 
cant is  applied  only  when  they  are  first  set  up  or  cleaned. 


CHAPTER  IV. 


OPERATION  AND  CHARACTERISTICS  OF  THE 
D.  C.  DYNAMO. 


(a)  Preliminary  Tests. 

Before  starting  a  dynamo  or  motor  which  has  not  been  recently 
operated,  it  is  desirable  to  make  an  inspection  embracing  the  fol- 
lowing points. 

(1)  Does  the  armature  turn  easily  with  perfect  clearance? 

(2)  Is  the  commutator  clean?    If  not,  clean  it  with  fine  sand- 
paper.    Do  not  use  emery  paper,  as  this  is  likely  to  leave  con- 
ducting material  bridging  across  the  insulation  between  the  bars. 

(3)  In   case  the  commutator   surface   is   much   worn   by  the 
brushes,  these  cannot  be  fitted  properly  to  the  surface;  and  the 
machine  will  have  a  tendency  to  spark  when  loaded.     In  this 
event,  remove  one  of  the  bearings,  take  out  the  armature,  and 
have  the  commutator  turned  down  in  a  lathe.    Great  care  must  be 
taken  not  to  injure  the  inductors  on  the  armature  surface.    When 
replacing  the  armature,  the  oil  rings  will  have  to  be  lifted  into 
position. 

(4)  The  commutator  surface  being  even  and  clean,  ascertain 
whether  the  contact  surface  of  every  brush  is  smooth  and  fits 
the  commutator  perfectly.     If  not,  lift  the  brush,  insert  a  strip 
of  sandpaper  beneath  it  with  the  cutting  side  against  the  brush, 
and  draw  the  paper  back  and  forth.     In  this  way  the  brush  may 
be  made  to  conform  perfectly  to  the  shape  of  the  commutator. 

(5)  Ascertain   that   the   brushes    are    set   by   the   rocker   ap- 
proximately in  their  neutral   position.      If  the   armature   wind- 
ings are  visible,  the  neutral  commutator  bars  can  be  ascertained 
as  those  immediately  connected  to  inductors  that  are  midway  be- 
tween  two   pole-faces.      In   case   the   armature   is   covered,   the 
neutral  position  can  often  be  approximately  determined  as  the 
middle  of  the  space  allowed  for  play  of  the  rocker. 

(6)  See  that  a  moderate  and  yet  sufficient  pressure  is  exerted 
4 


4O  THE;  MOTOR  AND  THE  DYNAMO 

by  the  springs  upon  the  brushes.     A  little  experience  will  enable 
an  operator  to  judge  of  this  quite  accurately. 

(7)  Is  there  sufficient  oil  in  the  bearings  or  oil-cups? 

(8)  Do  all  electrical  connections  within  the  machine  appear  to 
be  correct?     Whether   or   not   they   are   correct   cannot   always 
be  determined  without  operating  the  machine,  as  will  be  described 
presently.     In  the  case  of  the  motor,  particular  care  must  be  ex- 
ercised to  see  that  the  field  connections  are  in  good  shape  and  that 
the  shunt  field  circuit  through  its  controlling  rheostat  (resistance 
coils)    is  perfect.     Should  the   shunt  field  circuit  become  open 
when  the  motor  is  in  operation,  the  machine  may  be  destroyed. 

(9)  Before  starting  to  operate  the  shunt  dynamo,  it  should 
be  ascertained  that  the  external  circuit  is  either  open  or  not  set 
for  excessive  load,  otherwise  the  machine  will  not  build  up,  and 
that  the  field  rheostat  is  turned  to  the  point  of  highest  resistance. 

(10)  Before  closing  the  switch  feeding  a  motor,  it  should  be 
ascertained  that  the  handle  of  the  starting  box  or  controller  is 
in  the  proper  starting  position,  and  in  the  case  of  a  shunt  motor 
that  the  field  rheostat  is  turned  to  the  point  of  lowest  resistance. 

After  everything  has  apparently  been  put  in  order  up  to  this 
point  and  the  dynamo  has  been  started  and  is  being  driven  at  its 
rated  speed,  if  it  fails  to  build  up,  even  when  the  field  rheostat 
is  turned  to  the  point  of  lowest  resistance,  and  the  switch  to  the 
load  circuit  is  open,  this  may  be  due  to  any  one  of  several  causes. 
It  is  an  easy  matter  to  enumerate  a  long  list  of  so-called  diseases 
of  the  dynamo.  Experience,  however,  suggests  the  following 
methods  of  procedure,  taking  each  in  turn  until  the  fault  is  dis- 
covered. 

(n)  Shift  the  brushes  slightly  forward  and  backward  by 
means  of  the  rocker,  watching  the  voltmeter  meanwhile  to  observe 
any  tendency  toward  building  up. 

(12)  Slightly  increase  the  pressure  on  the  brushes,  which  in 
low-voltage  machines  may  be  done  with  the  hand,  watching  the 
voltmeter  meanwhile.    This  may  be  combined  with  paragraph  n, 
and  sometimes  reveals  a  faulty  brush. 

(13)  If  the  machine  still  fails  to  build  and  the  voltmeter  leads 


OPERATION   AND   CHARACTERISTICS   OF  THE  D.   C.  DYNAMO         4! 

are  assuredly  connected  and  the  right  way  around  showing  a 
readable  voltage,  then  open  the  shunt-field  circuit.  If  this  causes 
a  slight  increase  in  voltage,  it  is  a  sign  that  the  current  in  the 
field  coils  caused  by  the  residual  magnetism  is  in  a  direction  such 
as  to  reduce  this  magnetism  rather  than  to  build  it  up.  The 
leads  from  armature  to  shunt-field  must  therefore  be  reversed. 
Or  the  fault  may  be  corrected  by  driving  the  dynamo  in  the 
reverse  direction,  which  will  reverse  the  polarity  of  the  armature 
terminals. 

(14)  If  operation  No.  13  causes  the  voltmeter  needle  to  drop 
slightly,  it  is  a  sign  that  the  field  is  correctly  connected  to  the 
armature  terminals  and  the  fault  lies  elsewhere,  possibly  in  the 
field-coils    themselves.      Before    testing    out    these,   however,    it 
would  be  well,  slightly  to  increase  the  speed  of  the  machine,  if 
this   is  not  too  difficult.     A  slipping  belt  or   a  badly  governed 
prime-mover  is  sometimes  the  sole  cause  of  annoyance. 

(15)  If  operation  No.   13  causes  no  change  in  the  voltmeter 
reading,  it  is  probable  that  no  spark  appears  on  closing  and  open- 
ing the  field  circuit,  and  that  the  circuit  is  somewhere  broken, 
except  in  the  case  to  which-  paragraph  16  applies.     It  is  not  im- 
possible to  get  the  effect  noted  in  this  paragraph,  even  with  the 
presence  of  a  spark  on  opening  the  field  circuit,  if  just  half  of 
the  field   coils  should  happen  to  be   connected  the   wrong  way 
around,  giving  the  wrong  polarity  to  half  of  the  field  poles.    This, 
however,  is  not  at  all  likely  unless  the  field  has  been  taken  apart 
and  reassembled. 

(16)  It  sometimes  happens  that  the  residual  magnetism  of  the 
field  iron  disappears  or  becomes  reversed.     In  the  latter  case, 
the  machine   would  operate  perfectly,   the   polarity   of   the   ter- 
minals  alone   being  reversed,   making   the   switch-board   meters 
read  backwards.     In  the  case  of  lost  magnetism,  however,  the 
voltmeter  would  show  no  voltage  on  operation.     In  that  event 
paragraphs  13  to  15  would  not  apply.     Disconnect  one  armature 
lead  from  the  shunt  field  so  that  the  only  path  between  the  ter- 
minals of  the  field  will  be  through  the  field  coils  and  rheostat, 
and  by  means   of   wires   from   some  outside   source   of   similar 


42  THE  MOTOR  AND  THE  DYNAMO 

voltage  to  that  generated  by  the  machine  itself  send  a  current 
through  the  coils  for  a  few  seconds,  so  as  to  excite  the  field  and 
restore  the  residual  magnetism.  The  direction  of  this  current 
will  determine  the  future  polarity  of  the  machine,  but  will  not 
otherwise  affect  its  operation.  In  machines  of  say  over  25  kilo- 
watts capacity  great  care  must  be  exercised  in  opening  the  shunt- 
field  circuit  when  fully  excited,  as  the  inductive  voltage  so  pro- 
duced is  likely  to  pierce  the  insulation.  This  danger  can  be 
avoided  by  short-circuiting  the  field  through  a  rheostat  before 
removing  the  wires  of  the  charging  current.  In  case  the  dynamo 
is  one  of  220  or  550  volts,  sufficient  residual  magnetism  may 
usually  be  induced  in  the  field  by  a  no  volt  source  of  supply,  if 
the  higher  voltage  is  not  at  hand.  The  utter  loss  of  residual 
magnetism  by  the  field  iron  is  not  a  frequent  source  of  trouble. 

If  the  dynamo  still  fails  to  build  up,  the  trouble  is  likely  of  a 
serious  nature,  such  as  to  require  the  rewinding  of  field  or  arma- 
ture. As  a  means  of  locating  these  more  serious  faults,  the  fol- 
lowing tests  are  described.  Paragraphs  17,  18  and  19,  however, 
might  well  be  considered  a  part  of  the  original  inspection  of  a 
dynamo  and  the  test  should  be  made  on  new  machines  or  those 
which  have  been  recently  taken  apart  and  reassembled. 

(17)  Test  for  "grounds"  (a)  between  field  and  frame  and  (b) 
between  armature  and  core.  For  these  tests  connect  the  volt- 
meter and  machine  to  a  source  of  supply  of  similar  voltage  to 
that  of  the  machine,  according  to  Fig.  32. 

In  case  the  insulation  has  been  rubbed  off  one  of  the  windings 
at  any  point  so  that  the  bare  wire  lies  against  the  iron,  forming 
what  is  known  as  a  "grounding"  of  the  wire,  a  voltmeter  con- 
nected as  shown  on  a  no  volt  circuit  would  read  no  volts.  If 
the  voltmeter  reads  anything  less  than  the  circuit  voltage,  only  a 
partial  "ground"  is  indicated.  The  resistance  of  the  wire  cover- 
ing not  being  infinite,  the  voltmeter  will  always  read  something, 
sometimes  so  little,  however,  that  a  special  low-voltage  meter  is 
required  to  get  an  exact  reading.  If  the  resistance  of  the  meter 
be  known  (Rw),  the  resistance  of  the  ground  (R^)  may  be 
found  as  follows : 


OPERATION   AND   CHARACTERISTICS   OF   THE  D.    C.   DYNAMO 


43 


V/  =  line  voltage,  or  voltage  of  the  source,  and  let  V 
the  voltage  read.    Since  the  deflection  on  the  scale  is  proportional 
to  the  current  passing  through  the  meter,  the  instrument  becomes 
in  this  usage  an  ammeter,  and  we  may  write 

V,  V, 


whence 

and 

and 


v; 


Rw  '    B 

>,»  4-  R^ 

VV, 

—                  nr 

V, 

R,«          * 

U.  4-  R, 

V,RW  =  VR,« 

+  VR,, 

V 


The  insulation  between  field  and  frame  and  between  armature 
and  core  varies  greatly  with  the  size  and  type  of  machine  and 

V 


(a) 


Fig.  32. 


with  the  particular  voltage  for  which  it  is  wound.  According 
to  the  standard  rating  of  the  American  Institute  of  Electrical 
Engineers  (1902)  "The  insulation  resistance  of  the  complete 
apparatus  must  be  such  that  the  rated  voltage  of  the  apparatus 
will  not  send  more  than  1/1,000,000  of  the  full  load  current,  at 
the  rated  terminal  voltage,  through  the  insulation.  Where  the 
value  found  in  this  way  exceeds  i  megohm,  i  megohm  is  suffi- 


44  THE  MOTOR  AND  THE  DYNAMO 

cient."      Substituting  this   value   in  the   formula,   if   the  circuit 
voltage  is  no  and  the  voltmeter  have  a  resistance  of  20,000  ohms, 

V  R 

a  normal  value,    then    since   V  =  -  we  should  have 

(Rm  4-  Kf) 

T  T  O    NX'     2  O  OOO 

V  =  -  -  =  2.15  volts.    So  the  voltmeter  on  a  no  volt 

1,020,000 

machine  could  read  2.15  volts  sum  total  in  the  tests  just  described 
and  the  machine  still  come  within  the  rating. 

If  a  serious  ground  is  discovered  in  any  machine,  perhaps  the 
first  point  of  suspicion  is  the  binding  posts.  It  may  also  be  sug- 
gested here  that  by  separating  the  field  coils  from  one  another, 
each  may  be  tested  separately.  Should  a  serious  ground  be 
detected  at  two  different  points,  this  would  mean  that  part  of 
the  windings  are  made  inoperative,  a  large  part  of  the  current 
naturally  flowing  by  the  path  of  low  resistance,  namely,  the 
grounds. 

(18)  Taking   the   resistance   of   the   field   winding   sometimes 
reveals  a  fault,  and  in  any  event  is  a  desirable  preliminary  test 
on  any  machine.    It  can  be  easily  determined  by  taking  the  poten- 
tial difference  between  field  terminals  with  the  voltmeter  when  a 
known  current  is  flowing  through  the  coils.     By  baring  the  con- 
nections  between   the   spools,    the   drop   across   each   individual 
spool  may  easily  be  obtained.    In  case  any  spool  gives  a  markedly 
different  reading  from  the  others,  it  is  a  sign  that  the  winding 
has  been  injured  at  some  point,  causing  either  an  excessively  high 
resistance  or  a  short  circuit  within  the  spool.     The  resistance  of 
the  field  coils  can  be  more  accurately  determined  by  means  of  the 
Wheatstone  bridge,  and  the  current  used  will  not  be  sufficient  to 
heat  the  windings  and  change  their  resistance  during  the  test. 

(19)  Taking  the  armature  resistance  is  also  a  desirable  pre- 
liminary  test   on   any   dynamo   machine.      The   best   method   to 
employ  is  the  potential  difference  method  as  above.     The  arma- 
ture resistance  is  made  up  of  three  essential  parts,  namely,  the 
resistances  of  the  armature  winding,  of  the  brush  contacts  and 
of  the  leads  to  the  machine  terminals.     The  last  are  very  small 


OPERATION  AND  CHARACTERISTICS   OF  THE  D.   C.  DYNAMO         45 

and  may  be  included  in  that  of  the  brush  contacts.     The  method 
of  obtaining  these  values  is  indicated  in  Fig.  33. 


VARIABLE    RESISTANCE 


VWN/WV 


TOTAL  DROP 


Fig.  33- 


.00  ,30  #0 

Fig.  34.— Carbon  brush  resistance  curve. 

The  total  armature  drop  ought  to  be  small,  as  the  armature 
resistance  materially  affects  the  voltage  at  the  terminals  and  the 


46 


THE)  MOTOR  AND  THE  DYNAMO 


efficiency  of  the  machine.  The  brush  drop,  the  sum  of  the  two 
sides,  varies  from  about  1.2  to  about  2.8  volts.  It  is  greatly 
influenced  by  the  strength  of  the  current.  The  accompanying 
curve  (Fig.  34)  was  obtained  on  a  3  horse-power  interpole  motor 
of  the  Electro-Dynamo  Co.  The  current  used  in  obtaining  the 
resistance  of  the  armature  circuit  should  be  as  near  as  may  be 
to  the  full-load  current  of  the  machine.  The  armature  must 
not  be  allowed  to  rotate  as,  the  field  not  being  excited,  the 
machine  might  attain  a  dangerous  degree  of  speed. 

This  test  may  be  extended  in  the  following  way  in  order  to 
reveal  any  defect  in  the  armature  winding.  Hold  one  of  the 
voltmeter  leads  on  a  commutator  bar  in  contact  with  a  brush 
and  keeping  the  current  constant,  touch  the  other  lead  to  each 
succeeding  bar  in  turn.  The  increments  in  voltage  drop  should 
be  constant.  Any  increment  less  than  the  constant  indicates  a 
short  circuit  in  the  corresponding  winding-element.  A  zero 
increment  may  also  indicate  an  open  circuit  in  the  armature. 

(b)  The  Building-up  Curve. 

If  a  shunt  generator  with  the  field  circuit  open  be  brought  up 


Field   current 

Pig-  35.— Building-up  curve. 

to  constant  speed  and  the  field  circuit  be  then  closed,  the  ter- 


OPERATION   AND  CHARACTERISTICS   OF  THE  D.   C.  DYNAMO         4/ 

minal  voltage  of  the  machine  and  the  field  current  as  the  gen- 
erator builds  up  will  be  so  related  as  to  give  the  curve  of  Fig.  35. 
The  shape  of  this  curve  depends  upon  the  iron  of  the  magnetic 
circuit.  The  saturation  point  is  at  S.  Owing  to  the  hysteresis 
of  the  iron,  the  time  required  for  building  up  varies  from  a  few 
seconds  in  small  machines  to  a  minute  or  so  in  very  large  ones. 
The  curve  must  be  plotted  from  simultaneous  readings. 

(c)  Magnetization  Curve. 

This  curve,  showing  also  the  character  of  the  magnetic  circuit 
of  the  dynamo  machine,  is  easier  to  obtain  than  the  preceding. 
The  method  is  to  excite  the  field  from  some  outside  source,  a 
variable  rheostat  controlling  the  strength  of  the  exciting  current, 
and  to  read  the  voltage  developed  at  the  terminals  at  each  value 

of  the  field.    Then,  since  &  =  -       —r  ,  in  which  I  is  varied  at 

10  — 
sp 

will  and  tr^e  only  other  variable  is  /*,  depending  on  the  flux 
density  induced  in  the  iron,  it  follows  that  the  m.  m.  f  .,  which  is 

—  -  ,  varies  as  I.     Since  m.  m.  f.  is  also  HI,  the  field  current 
10 

I  plotted  as  abscissae  is  proportional  to  H.     (See  Chapter  II.) 


. 
Again  since  e.  m.  f  .  =    —  ^r,  —  in  any  dynamo  machine,  and  since 

for  any  one  machine  driven  at  constant  speed  the  only  variable 
in  the  second  member  of  this  equation  is  $,  it  follows  that  the 
terminal  voltage  varies  as  3>.  Now  since  <£  =  <J>s,  or  Bs,  when 
we  plot  the  e.  m.  f.'s  as  'ordinates,  we  are  plotting  values  pro- 
portional to  B.  Hence  the  curve  described  is  essentially  a  B  and 
H  curve,  and  the  only  reason  it  is  not  a  straight  line  is  because  of 
the  varying  values  of  /*.  See  Fig.  .36. 

The  lower  curve  is  drawn  with  gradually  increasing  values  of 
I,  the  upper  one  with  the  values  of  I  again  decreasing.  The  two 
curves  are  not  identical  because  of  hysteresis.  The  proper  work- 
ing part  of  the  curve  during  the  operation  of  the  machine  is 
somewhat  below  the  saturation  point,  say  about  the  region  no  v. 


48 


THE  MOTOR  AND  THE  DYNAMO 


It  would  not  be  economical  of  iron  to  operate  the  machine  much 
below  this  region  besides  other  disadvantages  of  having  the  iron 
poorly  saturated,  to  be  explained  later.  To  operate  the  machine 


.  ra    .  '2o 

/?/7?f>erfs 


Fig.  36. 


much  above  the  region  would  mean  poor  control  of  the  voltage 
generated,  even  if  greatly  varying  the  field  current. 

There  is  one  flaw   in  the  reasoning  of   the  preceding  para- 
graph,  and  the  curve  is  not  strictly  a  magnetic   curve.     This 


OPERATION   AND  CHARACTERISTICS  OF  THE  D.   C.  DYNAMO         49 

is  because  of  magnetic  leakage,  by  which  is  meant  the  stray 
field  extending  from  pole  to  pole  out  around  the  armature  and 
not  ever  cut,  therefore,  by  the  armature  inductors.  The  ratio 
of  the  total  flux  set  up  by  the  field  winding  to  that  part  of 
it  which  actually  passes  through  the  armature  iron  is  known  as 
the  coefficient  of  magnetic  leakage;  1.25  might  be  given  as  a 
normal  value  for  this  quantity,  although  in  small  machines  it  may 
exceed  this  and  in  very  large  multipolar  machines  it  is  often 
less. 

(d)  Armature  Reaction. 

In  a  generator  furnishing  current  and  in  a  motor  under  opera- 
tion another  factor  enters  in  to  interfere  with  the  flux  produced 
by  the  field  circuit.  This  is  the  magnetic  flux  due  to  the  current 
in  the  armature.  This  flux  may  be  divided  into  two  com- 
ponents: the  one  (A)  is  caused  by  the  cross  magnetization  of 
the  armature,  distorting  the  field  flux;  and  the  other  (B),  is 
caused  by  the  so-called  back  ampere  turns  in  the  armature  cir- 
cuit and  may  be  termed  a  reverse  magnetization  by  the  armature 
circuit,  weakening  the  flux  from  the  field. 

Figs.  37  and  38  show  how  these  two  effects  are  brought  about 
on  bipolar  machines. 


Fig  37.— Generator. 


The  part  of  the  armature  winding  represented  in  Fig.  37  (A) 
sets  up  a  cross  magnetization,  much  as  if  the  wires  formed  a 


5O  THE  MOTOR  AND  THE  DYNAMO 

continuous  vertical  helix,  the  north  pole  being  on  the  lower 
side  of  the  armature,  at  n.  If  the  magnitude  and  direction 
of  the  field  flux  be  indicated  by  arrow  F  (N  to  S  in  the  air),  and 
of  the  armature  flux  by  arrow  /  (s  to  n  in  the  iron),  then  R 
will  represent  the  direction  of  the  resultant  lines  of  force,  the 
field  flux  being  to  this  extent  distorted.  The  axis  of  commuta- 
tion, being  at  right  angles  to  this  resultant  flux,  is  given  a  lead 
from  the  line  of  symmetry  between  the  pole  faces  in  the  direction 
of  rotation  by  the  amount  0,  the  angle  of  lead.  This  lead  given 
to  the  brushes  causes  the  current  in  the  remaining  armature 
inductors  to  be  as  shown  in  Fig.  37  (B).  The  current  in  these 
inductors  creates  a  magnetic  flux  much  as  if  the  wires  formed  a 
continuous  horizontal  helix,  the  north  pole  being  on  the  right 
side  of  the  armature  at  n.  In  this  case  the  arrows  F  and  /  are 
directly  opposed  to  each  other,  the  result  being  a  weakening  of 
the  field  flux. 

In  Ampere's  rule  of  directions,  the  right  hand  applies  to  the 
generator  and  the  left  hand  to  the  motor.  Effect  (A)  in  the 
motor  is  therefore  just  the  reverse  of  the  generator  as  is  shown 
in  Fig.  38,  but  effect  (B)  is  the  same  in  both  machines.  In  the 


(B) 


Fig  38.  —Motor. 


motor  the  brushes  are  given  a  lagging  position  with  reference 
to  the  axis  of  symmetry  in  order  to  bring  them  into  the  neutral 
position. 

The  final  shape  of  the  field  flux  due  to  the  influence  on  it 


OPERATION  AND  CHARACTERISTICS   OF  THE  D.   C.  DYNAMO         51 

or  armature  reaction  is  shown  in  Fig.  39.  It  will  be  observed 
that  the  tendency  is  for  the  flux  density  to  be  increased  in  the 
trailing  pole-tip  of  the  generator  and  in  the  leading  pole-tip  of 
the  motor. 


Motor.  Generator. 

Fig-  39- 

In  designing  a  machine,  the  weakening  of  the  flux  caused  by 
armature  reaction  has  to  be  compensated  for  by  an  increase  in 
the  ampere  turns  of  the  field. 

(e)  External  Characteristics. 

Let  a  shunt  generator  be  driven  at  constant  speed  with  the 
field  rheostat  fixed  in  some  definite  position,  and  let  load  be  grad- 
ually added  to  the  external  circuit  by  means  of  a  bank  of  lamps 
in  parallel  or  by  a  variable  rheostat.  As  the  external  current  is 
increased,  the  voltage  at  the  machine  terminals  will  be  found  to 
fall.  This  is  due  to  three  causes. 

(1)  The  IR  drop  in  the  armature  nnnding.    As  the  load  cur- 
rent I  increases,  the  e.  m.  f.  required  to  send  it  through  the  con- 
stant armature  resistance  R  increases  in  the  same  ratio.     This 
e.  m.  f.  is  used  up  in  the  armature  and  is  subtracted  from  the 
generated  volts  thereby  rendering  the  terminal  volts  less. 

(2)  The   consequent   weakening   of   the   shunt   field    current. 
With  fixed  field  rheostat  the  resistance  of  the  field  circuit  is  es- 
sentially constant,  and  a  weakened  terminal  voltage  sends  through 
il  a  correspondingly  weaker  current.     This  lessens  the  flux  cut 
by  the  armature   inductors,   causing  a   still   further   voltage-de- 
crease.   After  each  addition  of  load,  this  interaction  is  set  up  and 
continues  till  a  balance  is  obtained. 

(3)  The  Armature  Reaction.     It  has  been  shown  in  the  pre- 


THE)  MOTOR  AND  THE)  DYNAMO 


ceding  section  (d)  how  this  phenomenon  decreases  the  field 
flux,  an  effect  which  is  almost  immediately  followed  by  a  fall 
in  terminal  voltage. 

Fig.  40  is  the  external  characteristic  of  a  2  horse-power  110- 
volt  Bell  motor,  operated  as  a  generator,  the  curve  being  plotted 
between  the  load  current  and  the  terminal  voltages.  The  part  of 
the  curve  which  returns  toward  the  origin  is  formed  when  the  de- 


.  LOUIS  CO. .New  York 


Fig.  40.— External  characteristic  of  a  shunt  generator.  Curve  A,  at  rated  speed  and 
field  current.  B,  at  rated  speed  and  increased  field  resistance.  C,  at  increased 
speed  and  still  greater  field  resistance. 

creased  resistance  in  the  external  circuit  so  reduces  the  terminal 
voltage  that  a  weaker  rather  than  a  stronger  current  flows  through 
this  decreased  resistance.  This  phenomenon  is  termed  in  power- 
house vernacular  "lying  down."  It  sets  in  with  a  very  unsteady 
condition  of  the  two  meters  and  is  accompanied  by  violent  spark- 
ing at  the  brushes. 


OPERATION   AND  CHARACTERISTICS  OF  THE  D.   C.  DYNAMO         53 

(f)  Armature  Characteristics. 
In  the  practical  operation  of  a  shunt  dynamo,  a  decrease  in 


Fig.  41.— Armature  characteristics  of  a  shunt  generator. 

terminal  voltage  due  to  load  can  up  to  a  certain  limit  of  output 
be  compensated  for  by  a  decrease  in  the  resistance  of  the  shunt 


54 


THE:  MOTOR  AND  THE:  DYNAMO 


field  rheostat.  This  increases  the  field  current  and  strengthens 
the  field  flux  to  such  a  degree  as  to  overcome  the  tendency  of 
the  terminal  voltage  to  decrease.  A  curve  plotted  between  load 
currents  and  field  currents  when  the  speed  and  terminal  voltage 
are  kept  constant  is  known  as  the  armature  characteristic.  For 
the  generator  of  the  preceding  section  operated  at  no  volts  and 
again  at  115  volts  constant  e.  m.  f .,  the  armature  characteristics 
are  shown  in  Fig.  41. 

(g)  The  Compound  Generator. 

From  the  external  characteristic  of  a  shunt  generator,  it  is 
at  once  apparent  that  in  order  for  such  a  machine  to  furnish 
constant  voltage  under  varying  loads,  the  services  of  an  attend- 
ant would  be  constantly  in  demand.  For  this  reason  various  auto- 
matic field  regulating  devices  have  been  invented.  By  far  the 


Atnp 


Fig.  42.— External  characteristics  of  compound  generators. 

simplest  and  most  practical  of  all  these  is  that  by  which  the  vary- 
ing load-current  is  carried  around  the  field  poles  on  what  is  known 
as  the  series  field  winding.  This  varying  load-current  then  adds 
its  m.  m.  f.  to  that  of  the  shunt  field  current,  thus  producing 
an  increase  of  flux  with  increase  of  load.  By  having  in  these 
series  field  coils  the  proper  number  of  turns,  the  terminal  e.  m.  f. 


OPERATION   AND  CHARACTERISTICS   OF  THE  D.   C.   DYNAMO         55 

of  the  machine  may  be  kept  constant  or  may  be  made  to  in- 
crease slightly  or  may  be  allowed  to  decrease  slightly  with  in- 
crease of  load  on  the  machine.  Thus  we  have  flat  compounding 
or  over  compounding  or  under  compounding  of  the  generator. 
A  simple  calculation  will  make  this  clear,  as  follows: 

From  an  inspection  of  Fig.  41  it  will  be  seen  that  for  a  load 
of  16  amperes  the  shunt  field  current  must  be  increased  from 
0.85  ampere  at  no  load  to  1.28  amperes,  an  addition  of  0.43 
ampere,  in  order  to  mantain  constant  terminal  voltage.  If  this 


Fig.  43. — Short  shunt. 


Fig.  44.— lyong  shunt. 

particular  field  winding  has  in  it  4,000  turns,  this  means  an 
increase  of  4,000  X  0.43  =  1,720  ampere  turns,  or  the  load 
current  of  16  amperes  must  encircle  the  poles  1,720  -^  16  =  108 
times  to  produce  the  required  additional  flux,  the  shunt  field 
remaining  constant  at  0.85  ampere.  For  a  bipolar  machine  this 
means  54  turns  to  each  pole  in  the  series  winding.  Fig.  42  rep- 
resents the  external  characteristics  of  a  compound  generator 
with  three  different  values  of  series  field  winding. 
5 


56  THE)   MOTOR  AND  THE)  DYNAMO 

In  order  to  overcome  the  IR  drop  that  always  occurs  in  line 
wires  of  any  considerable  length,  it  is  the  custom  to  build  gen- 
erators of  the  over  compounded  type,  and  then  by  inserting  a 
cross  shunt  of  the  proper  size  at  a  (see  Fig.  43)  to  regulate 
them  to  the  desired  degree  of  compounding.  There  are  two  styles 
of  connecting  the  shunt  field,  termed  short  shunt  and  long  shunt. 


8  ShunT  t-id'J  winding  . 
C  5*«>5  field  wirtctiriu 
A  fleulQtitn  :hunf 


Fig-  54-  —  Complete  field  frame  of  50  k.  w.  generator. 

There  is  very  little  difference  between  these  two  styles  of  connec- 
tion as  regards  the  behavior  of  the  machine. 

(h)  Sparking. 

The  cause  of  sparking  at  the  brushes  of  direct-current  gen- 
erators and  motors  and  the  means  of  obviating  the  same,  par- 
ticularly in  the  latter,  have  received  more  attention  from  manu- 
facturers and  have  given  origin  to  more  types  and  styles  of  ap- 
paratus than  any  other  feature  of  dynamo  electric  machines.  The 
chief  objections  to  sparking  are  (i)  the  little  electric  arc  burns 


OPERATION  AND  CHARACTERISTICS   OF  THE)  D.   C.  DYNAMO         57 

and  mars  the  edges  of  the  commutator  bars  and  so  increases  this 
form  of  trouble  and  (2)  the  counter  e.  m.  f.  of  the  little  arcs 
interferes  with  the  e.  m.  f .  of  the  generator  and  the  speed  of 
the  motor. 

A  study  of  Fig.  46  will  serve  to  explain  the  cause  of  sparking 
and  the  immediate  remedy  for  it. 

The  figure  represents  part  of  a  gramme  ring  armature  with  a 
commutator  bar  to  every  second  turn  about  the  ring,  the  brush 
being  placed  on  the  axis  of  symmetry  (x.v)  between  the  poles. 
An  application  of  Ampere's  rule  for  direction  reveals  the  course 
of  the  generated  current  through  the  windings  to  either  side 
of  this  axis,  as  indicated  by  the  small  arrow-heads,  the  rotation 


Fig.  46. 

being  clockwise.  Six  winding-elements,  a,  b,  c,  d,  e,  -f,  are  shown 
with  commutator  bars  i,  2,  3,  4,  5,  6,  7.  Winding-element  d, 
being  in  the  neutral  position,  has  generated  in  it  no  e.  m.  f.,  the 
elements  to  the  right  of  it  sending  their  current  into  the  brush 
through  bar  4,  and  those  to  the  left  of  it  sending  their  current 
into  the  brush  through  bar  5.  As  bar  4  leaves  the  brush,  this  cir- 
cuit on  the  right  will  be  broken  at  the  brush-tip,  causing  an 
electric  arc.  For  when  this  circuit  is  broken,  although  element  d 
has  no  e.  m.  f.  generated  in  it,  yet  it  will  not  readily  become 
a  path  for  this  current  from  the  right  like  a  sort  of  side-track,  as 
the  connections  would  indicate,  because  of  self-induction. 

Self-induction  or  inductance  is  a  property  of  electric  circuits 


58  THE:  MOTOR  AND  THE;  DYNAMO 

which  tends  to  oppose  any  change  in  the  current  of  the  circuit. 
It  is  particularly  strong  in  those  circuits  which,  because  of  their 
helical  shape  or  the  presence  of  iron  in  their  neighborhood, 
naturally  develop  a  magnetic  flux  when  they  carry  a  current. 
Any  change  of  this  current  and  flux  sets  up  a  counter  e.  m.  f. 
in  the  circuit,  which  retards  the  change  and  particularly  inter!"  ei  es 
with  a  sudden  reversal  of  the  current.  Now  winding-element 
d  has  a  moment  before  been  in  position  e  and  had  a  current  flow- 
ing in  it  from  the  left.  It  cannot  therefore  instantly  take  up  the 
current  from  the  right  and  so  divert  the  flow  entering  the  brush- 
tip  through  commutator  bar  4. 

Two  things  may  be  done,  however,  to  remedy  this  difficulty, 
as  follows:  First,  the  brush  may  be  made  of  high  resistance 
material  (carbon)  which  will  aid  the  narrowing  contact  between 
brush  and  bar  4  to  oppose  the  flow  of  current  by  this  path  and 
will  further  reduce  to  a  minimum  any  current  circulating  around 
through  the  brush  and  the  short-circuited  element  d.  Secondly, 
if  the  poles  be  moved  to  the  position  of  the  dotted  lines,  or  which 
is  the  same  thing,  if  the  brush  be  shifted  slightly  in  the  direc- 
tion of  the  rotation,  the  winding-element  at  d  will  come  under 
the  influence  of  the  next  pole  earlier  than  before,  and  the  flux 
from  this  pole  will  generate  in  it  an  e.  m.  f.  which  will  aid 
in  reversing  the  current  from  the  left  so  as  to  offer  an  un- 
obstructed path  to  the  current  from  the  right.  This  current 
will  thus  be  made  to  enter  the  brush  partly  or  wholly  from  be- 
hind, through  commutator-bar  5,  thus  obviating  any  arc  between 
the  tip  of  the  brush  and  bar  4.  More  will  be  said  under  the  sub- 
ject of  variable  speed  motors  of  the  means  employed  for  creating 
flux  for  reversal  of  the  current  in  the  short-circuited  armature 
coil.  The  theory  and  remedy  of  sparking  is  identical  in  the  case 
of  the  gramme  ring  and  the  drum-wound  armature. 

(i)  Operation  of  D.  C.  Shunt  Generators  in  Parallel. 

All  dynamo-electric  machinery  operates  with  greatest  effi- 
ciency near  its  point  of  full  load,  the  efficiency  being  lowest 
under  light  loads.  For  this  reason,  in  generating  plants  where 
the  load  is  subject  to  wide  variations,  it  is  more  economical  to 


OPERATION   AND  CHARACTERISTICS   OF  THE  D.   C.  DYNAMO 


59 


have  several  smaller  machines  which  can  be  run  one  or  more  at 
a  time  than  to  have  one  large  machine  which  for  several  hours  a 
day  would  be  loaded  to  only  a  small  part  of  its  capacity.  Hence 
the  necessity  of  parallel  operation. 

Fig.  47  shows  the  connections  for  a  pair  of  shunt  generators 
feeding  the  bus  bars  of  a  distributing  service.  When  the  two 
machines  are  running,  the  switches  shown  in  the  figure  being 
closed,  the  load  may  be  distributed  between  the  machines  by  the 


To  PRIME 
MOVER 


Fig.  47.— Shunt  generators  in  parallel. 

manipulation  of  the  field  rheostats.  Being  in  parallel,  the  ter- 
minal voltage  of  the  two  machines  will  necessarily  be  the  same, 
but  a  change  of  field  which  would  cause  an  increase  in  the  e.  m.  f. 
of  one  results  in  its  taking  on  more  of  the  load.  Its  ammeter  will 
show  that  it  is  furnishing  more  current,  and  the  load  on  the  other 
machine  may  be  so  reduced  that  its  ammeter  will  read  less  than 
zero,  signifying  that  this  generator  is  now  drawing  current  from 
the  bus  bars  and  is  being  driven  as  a  motor,  its  prime  mover 
acting  with  a  tendency  to  race.  Such  a  state  of  things,  should 
it  occur  in  a  generating  plant,  would  be  more  or  less  dangerous 


6o 


THE)  MOTOR  AND  THE  DYNAMO 


to  the  machinery,  there  being  a  tendency  on  the  part  of  some 
generators  to  become  overloaded  and  on  the  part  of  those 
driven  to  spark  violently  at  the  brushes.  For  the  brushes  would 
not  be  in  the  correct  motor  position.  The  opening  of  a  generator 
field  circuit  would  cause  such  a  condition,  and  very  easily,  since 
a  shunt  generator  operates  in  the  same  direction  as  a  motor,  when 
the  field  and  armature  connections  are  unchanged. 

If  generators  are  to  operate  together  in  parallel,  it  is  desir- 
able that  their  external  characteristics  should  be  similar.  Fig. 
48  represents  the  external  characteristics  of  two  shunt  generators 
that  differ  considerably.  For  convenience  the  current  values  are 

Volts 


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o  i< 

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Fig.  48- 

plotted  in  opposite  directions.  Suppose  the  total  load  to  be  600 
ampers  at  no  volts,  the  field  rheostats  of  the  two  generators 
having  been  adjusted  so  that  each  furnishes  half.  Let  the  total 
load  be  reduced  to  300  amperes,  the  field  rheostats  being  un- 
altered. The  result  will  be  a  rise  in  voltage  to  113  volts;  but  the 
external  characteristics  of  the  machines  being  unlike,  the  points 
on  the  curves  coresponding  to  the  new  load  and  voltage  show 
one  machine  furnishing  twice  as  much  current  as  the  other,  200 
amperes  to  100  amperes.  Should  the  load  be  reduced  to  zero, 
the  voltage  would  evidently  rise  to  about  117  at  the  point  where 
the  curves  cross,  and  generator  number  2  would  furnish  about 


OPERATION  AND  CHARACTERISTICS  OF  THE  D.   C.  DYNAMO         6l 

100  amperes  to  generator  number  i,  driving  it  as  a  motor.  Hence 
such  dissimilar  machines,  when  operating  in  parallel  under  a  vary- 
ing total  load,  would  require  constant  attention  at  their  field 
rheostats. 

In  a  power  plant  when  one  of  the  generators  is  to  be  removed 
from  service,  the  procedure  is  to  reduce  its  field  till  its  ammeter 
reads  zero.  The  switch  can  then  be  opened  without  in  any  way 
changing  the  loads  on  the  other  machines.  Similarly,  to  bring  an 
idle  machine  into  service,  start  its  prime  mover,  bring  it  up  to  rated 
speed,  increase  its  voltage  to  that  of  the  bus  bars.  (The  volt- 
meter of  the  switch-board  is  made  to  serve  for  any  machine  de- 
sired by  a  rotating  switch  shown  in  the  figure.)  The  switch 
can  then  be  closed  connecting  it  to  the  bars,  and  its  field  can  be 
adjusted  until  it  takes  its  share  of  the  load.  The  machine  switches 
are  not  intended  to  be  opened  when  any  current  is  flowing  through 
them  or  to  be  closed  when  conditions  are  such  that  current  would 
immediately  flow  through  them. 

( j)  Operation  of  D.  C.  Compound  Generators  in  Parallel. 

Except  for  the  peculiar  action  of  the  series  fields,  the  operation 
of  compound  generators  in  parallel  is  similar  to  that  of  shunt 
generators.  The  connections  for  two  machines  are  represented 
in  Fig.  49. 

Consider  first  the  machines  to  be  in  operation  without  the 
equalizer  bus,  only  the  outside  blades  of  the  three-pole  switches 
being  closed,  and  consider  G^  by  a  slight  increase  in  speed 
momentarily  to  take  on  more  load  than  G2.  The  result  will  be 
to  increase  the  series  field  of  G!  above  that  of  G2,  which  results 
in  increasing  the  e.  m.  f.,  thus  still  further  increasing  the  load  on 
G±.  The  load  on  G2  being  thus  decreased,  its  series  field  is  weak- 
ened, thereby  accentuating  this  effect  till  G2  is  actually  driven  as 
a  motor.  And  not  only  so,  but  because  of  the  reversed  direction 
of  the  current  in  its  series  field,  G2  acts  as  a  differentially  wound 
motor  (see  p.  77),  increasing  in  speed  the  more  current  it  draws. 

Two  compound  generators  in  parallel  are  thus  in  unstable 
equilibrium.  By  the  introduction  of  the  equalizer  bus,  however, 
the  current  through  the  series  fields  becomes  the  same  in  all 


62 


THE  MOTOR  AND  THE)  DYNAMO 


machines  so  connected,  the  resistances  of  the  series  fields  being 
the  same.  It  is  therefore  impossible  for  the  phenomenon  just 
described  to  take  place.  It  is  only  by  the  introduction  of  this 
equalizer  connection,  Fig.  49,  that  compound  generators  can  be 

SERVICE  Bus. 


L 


EQUALIZER  Bus. 


SERVICE  Bus. 


To  PRIME ~ 
MOVER 


To  PRIME 

MOVER2 


SH.  F. 
Fig.  49.— Compound  generators  in  parallel. 

operated  in  parallel.     If  the  machines  are  not  of  the  same  size, 
the  series  field  resistances  must  be  in  proper  proportion. 

(k)  D.  C.  Generators  in  Series. 

Direct-current  generators  are  operated  in  series  for  the  same 
purpose  that  batteries  are  joined  in  series,  namely,  to  obtain 
increased  voltage.  There  are  three  applications  of  this  method 
of  operation  in  general  use :  viz.,  in  the  Edison  three-^wire  system, 
in  the  use  of  boosters,  and  in  the  multi-voltage  power  systems. 

The  Edison  three-wire  system  is  a  device  for  saving  copper  in 
transmission  lines.  The  connections  are  shown  in  Fig.  50. 

The  current  in  the  middle  wire  is  at  every  point  the  algebraic 
sum  of  the  current  in  the  outside  wires.  The  saving  in  copper. 


•    OPERATION   AND  CHARACTERISTICS  OF  THE  D.   C.  DYNAMO         63 

if  all  three  wires  are  of  the  same  size,  is  62^/2  per  cent.,  proved 
as  follows.  Double  the  voltage  gives  the  same  power  in  watts 
with  half  the  current.  The  PR  loss  in  the  conductors  willf  be 
the  same  on  a  220  volt  system  as  on  a  no  volt  system,  if  R  be 
made  4  times  as  great,  or  the  cross-section  of  the  wire  %  as 
great.  On  a  two- wire  system  this  would  mean  a  saving  of  75 
per  cent,  of  copper  in  a  220  volt  system  as  against  a  no  volt 
system,  but  a  third  wire  of  the  same  size  as  the  other  two  adds  to 
the  25  per  cent,  used  half  again  as  much  copper,  or  in  all  37^ 
per  cent.,  leaving  the  economy  in  copper  a  62l/2  per  cent,  saving. 

ifO  A         30  A 3.0A 10  A 


OK)          (SlO                           QlO  QIC 

I  I   -»  *-| 2-1 


10  A 


30  A  iOA  10  A 

Fig.  50. — Edison  3-wire  system. 

In  connection  with  the  Edison  system  the  three-wire  generator 
ought  to  receive  a  mention.  This  is  a  220  volt  generator,  whose 
brushes  are  connected  to  the  two  outside  wires  of  the  system. 
Besides  the  commutator,  there  are  on  the  armature  shaft  two 
rings,  tapped  on  to  the  winding  as.  in  a  single  phase  alternating 
current  generator.  These  are  connected  through  a  highly  induc- 
tive circuit,  known  as  a  reactor,  whose  middle  point  leads  out  to 
the  middle  wire  of  the  system.  The  current  (direct-current)  in 
this  wire  flows  alternately  through  either  half  of  the  reactor 
circuit,  thus  forming  part  of  the  alternating  current  in  the 
armature. 

Boosters  are  low-voltage  generators  of  large  current  capacity 
used  in  series  with  the  main  generator  of  a  system  for  the  pur- 
pose of  stepping  up  the  voltage  a  few  points  on  special  branches 
of  the  main  system.  For  instance,  where  storage  batteries  are 
in  use  to  operate  in  parallel  with  a  generator,  as  is  sometimes 
done  in  systems  where  there  is  great  variation  in  load,  during 


64  THE)  MOTOR  AND  THE  DYNAMO 

those  hours  of  the  day  when  the  load  is  light  and  the  battery  is 
being  charged,  a  voltage  above  generator  voltage  must  be  applied 
to  the  battery  terminals  in  order  to  overcome  the  counter  electro- 
motive force  of  the  battery.  The  additional  volts  would  be 
obtained  by  means  of  a  booster.  Again  in  order  to  counteract  the 
IR  drop  in  line  wires,  the  feeder  system  as  illustrated  in  Fig.  51 
is  very  efficient  and  convenient. 

feeder 


Booster 


Fig.  51.— Booster-feeder  system. 

The  booster  need  be  only  a  fraction  of  the  size  of  the  main 
generator,  as  it  supplies  only  a  few  additional  volts  to  part  of 
the  load. 

Multi-voltage  power  systems  will  be  noticed  under  the  subject 
of  variable  speed  motors. 

(1)  D.  C.  Arc-light  Dynamos. 

The  open  arc-light  operates  best  at  45  volts  and  requires  from 
6.5  to  10  amperes.  Because  of  their  low  voltage  and  large  cur- 
rent such  lamps  came  to  be  arranged  in  series  groups,  and 
machines  were  devised  which  furnished  constant  current  at  a 
high  voltage  in  distinction  from  the  constant  voltage  generators 
for  incandescent  lighting  service.  These  machines,  however,  are 
now  rapidly  passing  out  of  use  for  two  reasons:  First,  the 
enclosed  arc  lamp  has  been  invented,  which  requires  about  75 
volts  at  the  arc  and  takes  5  amperes,  more  or  less,  so  that  with  a 
small  rheostat  in  series  with  each  lamp,  these  lamps  operate  very 
well  in  parallel  on  the  usual  no  volt  circuit.  Secondly,  where 
for  any  reason  series  lamps  are  preferred,  alternating-current 
service  has  so  many  advantages  over  direct-current  for  series 
lighting,  that  the  latter  is  being  rapidly  superceded.  Since,  how- 


OPERATION  AND  CHARACTERISTICS  OF  THE  D.   C.  DYNAMO         65 

ever,  such  circuits  are  still  occasionally  met  with,  it  may  be  best 
to  introduce  here  a  brief  description  of  some  types  of  constant 
current  high  potential  direct-current  generators. 

Since  the  lamps  are  in  series,  when  a  lamp  is  shunted  out  of 
service,  in  order  to  maintain  constant  currents,  the  generator  volt- 
age must  be  decreased,  and  vice  versa.  Two  ways  of  decreasing 
voltage  are  to-  decrease  the  field  circuit  by  means  of  a  rheostat 
or  to  shift  the  brushes  so  as  to  include  fewer  active  armature 
coils  between  them.  Either  of  these  methods  used  alone  causes 
violent  sparking  at  the  brushes,  as  is  the  case  in  the  old  Thomson- 
Houston  dynamo  which  employs  the  second  method.  The 
Excelsior  arc-lighting  generator  and  the  Brush  machine  use 
both  methods  combined  and  are  more  successful.  In  all  these 
machines  the  field  regulation  or  brush  shifting,  as  the  case  may 
be,  is  accomplished  by  an  automatic  device  more  or  less  compli- 
cated in  construction  and  adjustment.  A  fuller  description  of 
these  will  be  found  in  such  detail  works  as  Crocker's  "Electric 
Lighting,"  Vol.  I,  Sheldon  and  Hausmann's  "Dynamo  Electric 
Machinery,"  Vol.  I,  etc. 

In  conclusion  it  should  be  stated  that  there  are  numerous 
variations  of  the  types  of  dynamos  thus  far  treated,  some  to  be 
found  only  in  Europe,  such  as  the  disc  dynamo,  others  used  only 
as  motors,  which  will  be  taken  up  later.  The  underlying  prin- 
ciples of  all  these  are  not  different  from  those  described.  The 
greatest  divergence  of  design  occurs  in  the  case  of  alternating- 
current  generators,  which  will  be  treated  in  the  second  part  of 
this  volume. 


CHAPTER  V. 


THE  D.  C.  MOTOR. 


(a)  Operation  and  Characteristics. 

The   fundamental  equation  of  the  shunt  motor,  as  given  on 
page  15,  is 


~~          " 


where  E/,  the  line  voltage  applied  to  the  machine,  is  opposed 
by  the  counter  e.  m.  f.,  leaving  only  a  small  remnant  to  send  the 
working  current  Ia  through  the  low  resistance  of  the  armature 
circuit  Rrt.  For  instance,  in  a  certain  3  horse-power  (so  rated) 
shunt  motor,  115  volts,  25  amperes,  the  field  current  is  i  ampere 
and  the  armature  current  at  full  load  24  amperes.  The  armature 
resistance  is  0.45  ohms.  Now  the  IaRa  drop  =  24  X  0.45  = 
10.8  volts.  Hence  this  is  the  effective  pressure,  the  potential 
difference  required  to  send  24  amperes  through  the  armature 
circuit  when  the  machine  is  at  rest.  The  c.  e.  m.  f  .  developed  by 
the  rotating  armature,  that  is,  the  e.  m.  f.,  with  the  same  field 
and  speed  conditions  which  the  machine  would  develop  if  opera- 
ting as  a  generator  is  115  minus  10.8,  or  104.2  volts. 

When  the  machine  is  running,  this  c.  e.  m.  f  .  of  104.2  volts 
acts  like  a  resistance,  preventing  the  current  from  becoming 
excessive.  But  if  the  motor  when  at  rest  should  be  connected 
directly  to  the  115  volt  mains  without  any  starting  device,  the 

current  in  the  armature  would  be  --  =  2,555  amperes,   an 

°-45 

amount  which,  if  it  should  not  open  circuit-breakers  or  blow 
fuses,  would  burn  up  the  armature  circuit.  While  a  direct- 
current  motor  is  coming  up  to  speed,  therefore,  a  temporary 
resistance  is  thrown  in  between  the  armature  circuit  and  the  line, 
in  the  shape  of  a  starting-box.  The  shunt  field  circuit,  on  the 
other  hand,  has  in  itself  such  a  resistance  that  it  can  be  con- 
nected directly  to  the  mains  without  injury. 


THF,  D.   C.   MOTOR  67 

The  connections  of  the  ordinary  starting-box  are  represented 
in  Fig.  53. 

When  the  switch  S  has  been  closed,  the  handle  of  the  starting- 
box  is  slowly  moved  from  stud  to  stud,  cutting  out  the  resistance- 
coils  R,  till  the  left-hand  lead  is  directly  connected  to  the  arma- 
ture and  field  like  the  right-hand  lead.  These  resistance-coils, 
although  at  first  interposed  in  the  field  circuit,  have  no  appre- 


Fig.  52. — G.  E.  Co.  50  h-p.  motor  with  starting-box  and  circuit  breaker. 

ciable  effect  on  the  shunt  field  current,  because  their  resistance 
is  infinitesimal  as  compared  to  that  of  the  field.  In  some  boxes, 
the  field  current  does  not  pass  through  these  coils  at  all.  M  is 
simply  a  retaining  magnet,  or  no-voltage  release,  for  the  handle. 
When  the  motor  is  to  be  stopped,  the  switch  S  is  pulled,  M  loses 
•its  magnetism,  and  the  handle  flies  back  by  means  of  a  spring  to 


68 


THE:  MOTOR  AND  THE;  DYNAMO 


Fig-  53-— Ordinary  shunt-motor  starting  box  and  connections. 


Fig.  54.— Starting-box  for  shunt  motor  with  no-voltage  release.     G.  E.  Co. 


THE  D.   C.   MOTOR 


its  original  position,  ready  to  be  used  again.     Other  forms  of 
starting  device  will  be  treated  later. 

In  Chapter  II  we  had  the  formula  W  =  i&,  where  W  =  ergs 


Fig-   55-— Starting-box,  interior.    G.  E.  Co. 


Fig.  56.— Starting-box  with  no-voltage  and  overhead  release.     G.  E.  Co. 

performed  when  a  wire,  having  current  i  absolute  units,  is  moved 
by  magnetic  influence  so  as  to  cut  $  lines  of  force.  This  formula 
can  be  developed  so  as  to  give  an  expression  for  the  torque  T 


/O  THE   MOTOR  AND  THE  DYNAMO 

or  twisting  force  of  the  armature  of  an  electric  motor  as  follows. 
The  total  flux  cut  per  revolution  by  each  armature  inductor  is 
the  $/>  of  the  fundamental  equation  of  the  motor,  which  takes 
the  place  of  the  3>  in  the  above  formula.  Similarly  i  must  be 

*  N 

replaced  by  ~j-  where  N  is  the  number  of  armature  surface- 
inductors,  as  before,  and  p'  is  the  number  of  paths  in  parallel, 
4  being  the  total  armature  current,  Irt,  in  absolute  units.  The 
work  in  ergs  per  revolution  of  360°  or  2ir  radians  (angular 
measure)  =  2-n-T,  when  T  is  the  force  in  dynes  operating  at  the 
end  of  a  radius  of  I  centimeter.  Hence 

T  - 

To  express  this  torque  in  pounds  developed  at  the  end  of  a 
foot  radius,  the  usual  practical  torque  unit,  the  following  changes 
are  necessary. 


i  ft.                   Amps. 

<!>/>N            j 

I 

,  •          A                 A                 .  . 

p                           10                  27T        X         2-54 

cm.  per  in. 

X     453-6     X   980   X 

gms.  per  Ib.       dynes          i 

12    ' 

per  gm. 

or  T  =  — ^r~j-Ia  X  o.  1 175  where  Ia  is  in  amps. 
io/> 

Now  since  power,  P,  is  work  per  second,  Wn,  we  have 


P  (in  ergs  per  sec.)  =  2vnT  =     -^— ia  absolute. 

To  reduce  to  watts,  or  io7  ergs  per  second,  and  to  amperes, 

=  volts.  X  Amps 

3>/>Nrc  3>/>Nrc  Volts  f 

P(m  watts)  =     ^g         X  ia  X  io  =      '          ia  =  c.e.m.f.  X  L 

which  is  simply  another  form  of  Lenz's  law. 

The  interpretation  of  the  equations  of  the  motor  will  make 
clear  the  characteristics  of  the  machine.  In  the  first  place  in 
the  equation  K/  —  c.  e.  m.  f,  -{-  I(IRa- 

E/  and  R^  are  essentially  constant.  Consider  now  .a  load  to 
be  thrown  on  the  motor,  as  happens  when  it  is  made  to  drive 
machinery.  The  decrease  in  speed  due  to  the  load  decreases  the 


THE  D.  c.  MOTOR  71 

c.  e.  m.  f  .,  as  is  evident  from  the  formula  for  the  same,  and  the 
result  is  an  increase  in  Ia.  This  means  an  increase  in  torqu 
This  increase  in  current  and  torque  is  more  rapid  than  the  accom- 
panying decrease  in  speed,  and  increases  automatically,  the  greater 
the  load  put  upon  the  motor.  It  is  therefore  unnecessary  to 
feed  into  a  motor  by  rheostat  control  or  otherwise  more  or  less 
current  according  to  the  power  desired,  for  if  the  e.  m.  f.  of  the 
supply  mains  is  kept  constant,  the  motor  will  draw  whatever 
current  it  needs  to  meet  the  load.  In  the  direct-current  motors 
indeed  it  is  possible  to  overload  the  machine  to  such  a  point  that 
the  load  current  will  overheat  and  destroy  the  armature.  In 
this  way  a  motor  may  be  made  to  furnish  many  times  its  rated 
power,  the  current  capacity  of  the  windings  alone  determining 
the  limit  of  power. 

Another  thing  which  the  motor  formulae  make  clear  is  the 
fact  that  a  decrease  in  the  field  current  of  a  shunt  motor  increases 
the  speed.  This  is  the  most  common  method  of  speed  control 
for  such  motors  and  is  effected  by  means  of  a  rheostat  inserted 
in  the  field  circuit,  exactly  similar  in  many  cases  to  the  rheostat 
used  to  control  the  voltage  in  shunt-wound  generators.  From 


formula  H/  =  -  I^R*  it  is  evident  that  if  a  shunt  motor 


be  furnishing  a  given  torque  and  its  field  current  is  decreased,  3> 
will  be  made  smaller  and  the  c.  e.  m.  f.  therefore  also  smaller. 
This  will  cause  an  increase  in  Ilt  and  the  machine  will  speed  up. 
This  increased  value  of  n  will  operate  to  counteract  the  decrease 
in  3>,  and  a  new  balance  will  be  obtained  between  the  impressed 
volts  E/  on  the  one  hand  and  the  c.e.m.f.  plus  armature  drop  on 
the  other.  If  the  torque  remains  constant  throughout  the  opera- 
tion, since  P  varies  as  nT,  the  new  point  of  equilibrium  will  show 
an  increase  in  developed  power  over  the  old,  the  decrease  in  4> 
not  being  quite  compensated  for  by  the  increase  in  n,  and  the 
new  value  of  Ia  being  therefore  greater  than  the  original  value. 
If,  however,  the  torque  demanded  of  the  motor  be  so  decreased 
wth  increase  of  speed  as  to  keep  the  power  constant,  then  the 
increase  in  !„  will  be  only  momentary,  the  new  speed  almost 
6 


72  THE  MOTOR  AND  THE  DYNAMO 

exactly  compensating  for  the  decrease  in  <£  so  that  the  c.  e.  m.  f . 
is  kept  constant. 

From  this  discussion  it  will  at  once  be  evident  that  an  increase 
in  the  load  on  a  shunt  motor  is  always  accompanied  by  a  slight 
decrease  in  speed  unless  special  means  are  taken  to  prevent  it. 
For  when  la  increases,  the  impressed  voltage  remaining  constant, 
the  field  current  remains  constant,  and  therefore  the  principal 
agent  in  effecting  the  necessary  decrease  in  the  second  member 
of  our  equation  is  n.  In  actual  operation,  though  the  field 
remain  constant,  3>  is  not  absolutely  unchanged  with  increase  of 
Ia.  The  armature  reaction,  noticed  under  the  characteristics  of 
generators  on  page  50  lessens  the  field  flux  in  motors  as  well, 
and  so  acts  like  a  resistance  in  the  field  circuit.  For  this  reason 
the  -speed  does  not  fall  off  as  much  as  it  otherwise  would  with 
increase  of  load.  In  fact,  it  is  possible  in  some  machines  to  set 
the  brushes  in  such  a  way  that  the  speed  will  not  decrease  at 
all,  or  may  even  increase.  This  effect  would  be  brought  about 
by  an  extreme  backward  lead  of  the  brushes.  It  is  usually 
accompanied,  however,  by  a  decrease  in  efficiency  and  by  danger 
of  sparking,  and  is  therefore  not  usually  resorted  to. 

In  the  case  of  the  series  motor,  where  the  armature  and  field 
current  are  necessarily  the  same,  the  increase  of  load  on  the 
machine  brings  with  it  an  increase  of  field  flux  <£,  and  hence 
a  far  greater  decrease  in  n  than  in  the  case  of  the  shunt  motor. 
With  this  increase  in  3>  and  Ia  there  comes  also  a  greater  increase 
in  torque  than  in  the  shunt  machine,  for  torque  varies  as  3>  X  I. 
It  is  the  peculiar  characteristic  of  a  series  motor  to  show  great 
changes  of  speed  under  varying  conditions  of  load,  and  at  the 
low  speeds  to  develop  a  very  high  torque.  For  this  reason  series 
motors  are  particularly  adapted  to  purposes  of  traction.  When 
a  car  of  any  sort  is  starting,  the  torque  to  overcome  the  static 
friction  must  be  large  and  the  speed  low.  After  the  inertia  of 
the  mass  has  been  overcome,  the  little  power  required  to  main- 
tain motion  on  a  level  track  or  road  reduces  both  Ia  and  $,  hence 
the  great  increase  in  -speed,  n.  The  shunt  motor,  on  the  other 
hand,  is  well  suited  to  operate  machinery  of  nearly  every  type, 


THE  D.   C.   MOTOR 


73 


approximately    constant    speed    under    varying    loads    being   the 
usually  desired  condition  of  operation. 

A  favorite  method  of  obtaining  the  characteristic  curves  of 
motors   is   by  means   of   the   friction   brake   or   other   form  of 


n 


n 


Fig.  57.— The  friction  brake. 

dynamometer.     Fig.  57  represents  a  convenient  form  of  such  an 
apparatus. 

W  is  a  piece  of  heavy  cotton  webbing  placed  about  the  pulley- 
wheel,  as  shown.  B  is  a  spring-balance  for  regulating  the  degree 
of  tension  by  means  of  the  hand-wheel  and  screw  S.  L  is  a 


Fig.  58-—  Cross-section  of  wheel  for  brake. 


steel-yard  or  beam-balance.     The  difference  in  reading  between 
L  and  B  is  the  pull  exerted  by  the  motor  at  the  rim  of  the  wheel, 


74  THE;  MOTOR  AND  THE  DYNAMO 

and  this  number  multiplied  by  the  circumference  is  the  work 
per  revolution.  In  getting  the  circumference  of  the  wheel  the 
radius  is  taken  to  the  middle  of  the  strip  of  webbing.  The  wheel 
itself  is  preferably  larger  than  the  pulley-wheel  usually  fur- 
nished with  the  motor,  and  the  surface  should  be  flat,  not 
crowned.  It  is  also  well  to  use  a  wheel  of  special  form,  whose 
cross-section  is  shown  in  Fig.  58.  This  is  capable  of  holding 
water,  which  may  be  replaced  from  time  to  time  during  the  test, 
keeping  the  wheel  cool.  This  insures  greater  constancy  of  fric- 
tion and  prevents  the  heat  developed  by  the  brake  from  being 
conveyed  through  the  shaft  into  the  bearings.  The  formula  for 
power  developed  by  the  motor  is  then 

Lbs.  X  ft.  circumference  X  r.p.m. 
Horse-power  —  -  -  . 

33,000 

This  is  the  output.  The  input  in  watts  may  be  obtained  by  an 
ammeter  in  the  general  circuit  (field  and  armature)  and  a  volt- 
meter across  the  terminals.  Horse-power  input  is  the  watts  di- 
vided by  746,  and  the  per  cent,  efficiency  is 

output 

-T—       -  X  loo. 
input 

(b)  Varieties  of  Field  Excitation. 

Fig.  59  shows  the  characteristic  curves  of  a  Bell  Electric 
Company's  shunt  motor,  rated  at  3  horse-power,  115  volts,  25 
amperes  1,200  revolutions  per  minute.  Fig.  60  shows  the  char- 
acteristic of  a  General  Electric  Company's  crane  motor  (series), 
rated  at  5  horse-power,  220  volts,  25  amperes  full  load. 

From  the  formula  E/  =  c.  e.  m.  f.  -f-  IrtR«  it  is  evident,  that 
the  smaller  the  resistance  of  the  armature  circuit,  the  more 
nearly  constant  will  be  the  speed  of  a  shunt  motor  under  vary- 
ing loads.  For  the  smaller  the  changes  in  IrtR.T,  the  more 

/        3>/>N#   \ 
nearly  will  c.  e.  m.  f.  (  =  —    8  ,     1  approach  a  constant  value.     In 

the  actual  operation  of  nearly  every  motor,  on  the  other  hand, 
it  must  not  be  overlooked  that  the  speed  variation  is  consider- 
ably greater  than  appears  from  the  manufacturer's  curves.  The 


THE  D.   C.   MOTOR 


75 


reason  is  the  IR  drop  is  always  present  to  a  greater  or  less  de- 
gree in  the  live  wires  leading  to  the  machine  from  the  source 
of  supply.  Unless  the  generator  is  compounded  for  this  particu- 


H-P -Output 

Fig-  59.— Curves  of  a  shunt-motor.    Bell  Electric  Motor  Co.    3  H.  P. 

lar  circuit  or  unless  some  other  equally  efficient  means  is  adopted 
to  maintain  constant  load-voltage,  this  IR  drop  lowers  the  voltage 
at  the  motor  as  the  load  increases,  and  has  to  be  reckoned  with  in 
considering  motor  speeds. 


THE;  MOTOR  AND  THE;  DYNAMO 


Again,  the  speed  of  a  motor,  both  shunt  and  series,  is  not  the 
same  after  it  has  been  operating  for  half  an  hour  as  at  first. 
The  heating  of  the  field  increases  its  resistance  and  so  decreases 


Fig.  60.— Curves  of  a  series  motor.     G.  E.  Co.    5  H.P. 

the  flux.     This  results  in  a  rise  in  speed  which  may  be  as  high 
as  4  or  5  per  cent. 

Shunt  motors  may  be  compounded  like  generators,  the  char- 
acteristics of  such  machines  resembling  those  of  the  shunt  mo- 
tor, but  inclining  toward  those  of  the  series  motor  in  shape.  An 


THE  D.  c.  MOTOR  77 

interpretation  of  the  formula  of  the  compound  motor  will  make 
this  clear,  namely, 

E/  =  c.  e.  m.  f.  +  IaRa  +  I,R, 

where  ISRS  is  the  IR  drop  in  the  series  field  winding.  Since  Ia 
and  1^  are  equal,  the  equation  may  be  written 

E  =  c.  e.  m.  f.  +  Ifl(Rffl+  R,), 

which  shows  that  the  series  field  is  equivalent  to  an  added  re- 
sistance in  the  armature  circuit.  Hence  the  speed  variation  in 
such  motors  is  larger  than  in  the  shunt  motor.  But  not  from 
this  cause  alone  is  this  true.  I?  in  a  compound  wound  motor  goes 
to  increasing  the  field  flux,  <£,  rendering  a  still  greater  decrease 
in  n  necessary  to  reduce  the  c.  e.  m.  f .  so  as  to  balance  the  above 
equation  than  would  be  required  in  the  case  of  the  shunt  motor. 
On  the  other  hand,  the  increase  in  3>  with  load  increases  the 
torque,  so  that  a  compound  motor  not  only  starts  more  slowly 
than  the  same  machine  would  without  the  compound  winding, 
but  exerts  a  greater  torque  at  starting.  It  is  therefore  suited 
for  those  cases  where  an  essentially  constant  speed  motor  is 
desired,  but  one  that  is  capable  of  meeting  the  requirements 
of  a  widely  varying  load.  This  is  the  case  in  the  operation  of 
passenger  elevators.  For  derricks,  on  the  other  hand,  and  mine- 
hoists,  where  the  speed  variation  is  of  minor  importance,  the 
series  motor  is  more  serviceable. 

Given  a  compound  generator,  Fig.  61,  to  be  operated  as  a  com- 
pound motor,  the  series  field  connections  must  be  reversed  as 
in  Fig.  62,  else  the  series  current  will  flow  in  a  direction  to  op- 
pose the  shunt  field. 

If  the  generator  of  Fig.  61  should  be  operated  as  a  motor, 
without  reversing  the  series  field  terminals,  it  would  be  what 
is  known  as  a  differential  motor,  the  series  field  acting  so  as 
to  reduce  the  flux  with  increase  of  load.  The  result  is  a  less 
falling  off  in  speed  than  when  operated  as  a  shunt  motor,  that  is, 
without  the  series  field.  In  fact  the  automatic  flux  reduction  by 
this  means  may  be  sufficient  to  increase  the  speed  with  increase 
of  load.  Operation  of  a  motor  under  such  conditions  is  a  matter 
of  great  risk,  as  the  speed  may  rise  to  a  dangerous  degree. 


7o  THE  MOTOR  AND  THE  DYNAMO 

Although  the  differential  machine  shows  a  slightly  lower  effi- 
ciency than  the  others,  yet  it  would  be  serviceable  where  abso- 
lutely constant  speed  is  demanded.  The  differential  motor  is, 
however,  little  used. 

From  this  discussion  of  the  motor  there  ought  not  to  be  omitted 
a  warning  against  loose  shunt-field  connections.     When  a  motor 


Fig.  61.— Compound  generator. 


Fig.  62  —Compound  motor. 

is  running  free  or  only  lightly  loaded,  the  opening  of  the  shunt 
field  circuit  results  in  a  sudden  decrease  of  the  field  flux  almost 
to  zero.  The  result  is  an  inrush  of  current  and  an  enormous 
increase  in  speed,  so  great  and  sudden  in  fact,  that  unless  a  fuse 
is  blown  or  an  automatic  circuit-breaker  opens  in  the  line,  the 
armature  will  fly  to  pieces  simply  by  centrifugal  force.  Not  too 
much  care  can  therefore  be  taken  to  have  all  connecting  points  in 
the  current  path  of  the  shunt  field  winding  secure. 


THE  D.   C.   MOTOR 


79 


(c)  Variable  Speed  Motors. 

The  control  of  speed  of  a  shunt  motor  by  means  of  resistance 
in  the  field  circuit  has  been  mentioned  on  page  71.  The  per- 
centage decrease  in  field  required  to  bring  about  a  given  increase 
in  speed  depends  on  the  magnetization  curve  of  the  machine 
combined  with  armature  reaction.  This  latter  becomes  greater 
and  greater,  distorting  the  field  flux  more  and  more,  the  weaker 
the  flux  becomes.  Fig.  63  represents  the  distribution  of  flux  in 
a  motor  field  (a)  with  strong  field  excitation  and  (b)  with  reduced 
excitation  for  increase  of  speed,  both  being  under  condition  of 
full  load  on  the  motor. 

Such  field  distortion  naturally  leads  to  sparking  (see  page  63) 


pole 


0" 


po  \< 


(A) 


(B) 


Fig.  63. 


even  when  the  brushes  are  shifted  to  the  new  neutral  axis. 
Sparking  sets  the  limit  of  speed  increase  by  this  method. 

The  ordinary  type  of  shunt  motor  with  simplex-wound  arma- 
ture can  stand  a  speed  increase  of  only  about  30  per  cent.,  for 
which  a  field  decrease  of  about  50  per  cent,  is  required.  Numer- 
ous means  have  been  devised,  however,  for  overcoming  spark- 
ing, so  that  now  a  speed  range  of  from  i  to  6  is  successfully 
obtained  in  shunt  motors.  Underlying  these  various  devices  of 
different  inventors  and  manufacturers  there  are  but  two  funda- 
mental principles  to  be  observed.  The  one  is  the  reduction  of 
the  self-induction  of  the  armature  circuit  to  a  minimum,  the 
other  is  the  prevention  of  the  distorting  effect  of  armature  reac- 
tion on  the  field  flux. 

Self-induction  in  an  electric  circuit  varies  as  the  square  of  the 


8O  THE  MOTOR  AND  THE  DYNAMO 

number  of  turns.     This  will  be  evident  if  we  consider  two  for- 
mulae of  Chapter  II,  namely, 

4?rNI  d& 

reluctance   '  at 

The  flux  $  in  this  case  is  not  the  field  flux,  but  a  flux  due  to 
the  current  in  those  armature  coils  in  which  commutation  is  tak- 
ing place.  They  are  the  coils  short-circuited  by  the  brush.  It 
is  the  dying  out  and  rebuilding  in  the  reverse  direction  of  this 
flux  which  causes  the  inductive  e.  m.  f .,  and  hence  the  spark  at 
commutation. 

In  self-induction,  where  the  same  turns  of  wire  produce  the 
flux  as  cut  the  flux,  a  doubling  of  N  means  a  quadrupling  of  e, 
or  the  e.  m.  f.  of  self-induction  varies  as  N2. 

If  therefore  each  armature  winding-element  be  made  to  have 
half  the  number  of  turns  and  the  number  of  winding-elements 
be  doubled,  the  self-induction  will  be  greatly  lessened.  This 
arrangement  necessitates  that  the  commutator  be  made  to  have 
twice  the  number  of  bars,  so  as  to  accommodate  the  increased 
number  of  winding-elements. 

Another  thing  that  aids  in  decreasing  self-induction  is  to  have 
the  current  in  each  armature  inductor  comparatively  small.  This 
can  be  brought  about  by  making  the  armature  winding  duplex 
or  triplex,  which  causes  the  current  to  be  shared  by  two  or  three 
windings.  This  again  increases  the  number  of  winding-elements 
and  of  commutator  bars.  It  also  requires  a  wider  brush,  two 
and  three  bars  being  covered  by  the  brush  face,  according  to  the 
winding.  To  be  sure,  both  of  these  features  of  the  winding  call 
for  a  larger  armature  core  than  usual,  and  also  a  larger 
commutator. 

The  increased  width  of  brush  necessitated  by  the  above-men- 
tioned features,  lengthens  the  period  during  which  the  coil  is 
short-circuited  by  the  brush,  that  is,  the  period  of  current  reversal 
in  the  coil — another  aid  in  reducing  the  self-induction,  as  appears 

from  e  =  —rr- 
at 

The  increased  size  of  armature  calls  for  a  larger  size  of  field 


THE  D.   C.   MOTOR  8l 

• 

than  common,  hence  the  gain  in  convenience  of  speed  control  is 
in  a  measure  offset  by  unwieldiness  and  expense  of  machine. 
In  general  practice,  almost  any  ordinary  shunt  or  compound 
wound  motor  in  which  the  brushes  are  made  to  overlap  two  or 
three  commutator  bars  will  be  found  capable  of  a  i  to  2  speed 
range,  if  not  more,  by  means  of  field  rheostat  control.  Greater 
speed  ranges  than  this,  whatever  other  means  may  be  used  to 
prevent  sparking,  call  for  motor  frames  as  follows : — 

Power  of  motor.  Size  of  frame. 

3  horse-power  5       horse-power 

5  horse-power  7  '/6  horse-power 

10  horse-power  15       horse-power 

The  Bullock  Mfg.  Co.  resorts  to  lengthening  the  armature  as 
well  as  increasing  the  diameter,  whence  the  unusual  size. 

As  regards  the  means  of  preventing  the  distorting  effect  of 
armature  reaction  on  a  weak  field,  several  manufacturers  resort 
to  a  special  shape  of  pole-piece.  The  Stow  motor,  instead  of 
changing  the  field  current,  has  movable  iron  plungers,  forming 
the  centers  of  the  field  cores  and  operated  by  gear-wheels.  An 
increased  air  gap  increases  reluctance  and  lessens  flux.  A  speed- 
range  of  I  to  4  is  obtainable  in  these  machines,  but  the  gearing 
is  unwieldy  and  expensive. 

In  order  to  avoid  the  extreme  distortion  caused  in  a  weak 
field  by  armature  reaction,  the  Fort  Wayne  motor  makes  use  of 
a  divided  field  core.  See  Fig.  64.  By  this  means  the  one-half 


Fig.  64.  — Pole  piece  of  Fort  Wayne  motor. 


of  the  field  core  is  kept  fairly  saturated  so  that  the  flux  from  the 
other  half  may  not  so  readily  be  crowded  into  it  by  armature 
reaction.  The  flux  curve,  then,  is  somewhat  as  shown  in  Fig.  65 
under  weak  field. 


82 


THE;   MOTOR  AND  THE  DYNAMO 


The  newest  type  of  Storey  motor  goes  a  step  further  in  this 
direction,  and  allows  of  no  direct  magnetic  connection  between 
the  two  halves  of  field  core,  the  encircling  binding-ring  being 
made  of  brass. 

In  Fig.  38  (A)  it  was  shown  how  the  current  in  part  of  the 
armature  inductors  creates  a  cross  magnetization,  distorting  the 
field  flux.  The  attempt  has  been  made  to  counteract  this  arma- 


poU 

I        n 


Fig.  65. 


ture  reaction  by  means  of  a  compensating  winding  imbedded  in 
the  pole  faces  and  connected  in  series  with  the  armature  so 
as  to  carry  the  armature  current,  only  in  a  reverse  direction. 
(Fig.  66.) 


Fig.  66. — Compensating  winding. 

This  corrective  device  was  not  found  sufficient  to  prevent 
sparking  with  weak  fields  and  furthermore  proved  expensive  in 
manufacture.  It  also  interfered  with  cooling. 

The  most  recent  and  by  far  the  most  successful  device  for 
counteracting  the  effects  of  armature  reaction  is  found  in  the 
interpole  motor,  for  whose  invention  credit  must  be  given  to 


THE  D.    C.   MOTOR 


Mr.  Pfatischer  of  the  Electro  Dynamic  Co.  Fig.  68  shows  such 
a  motor  of  this  company  and  Fig  69  the  same  machine  with 
armature  and  end-plate  removed.  The  interpoles  carry  a  wind- 
ing of  a  few  turns  connected  in  series  with  the  armature  circuit. 


Figs.  67.— Curves  of  an  interpolar  motor,  at  high  speed.    Electro  Dynamic  Co. 

Their  function  is  to  inject  into  the  short-circuited  armature 
inductors  at  the  instant  of  commutation  just  the  requisite  flux 
for  reversing  the  current.  The  excitation  of  the  interpoles,  being 


84 


THE  MOTOR  AND  THE  DYNAMO 


Fig.  63.— Interpole  motor— commuter  end.    Ball  bearings 


Fig.  69. — Interpole  motor.    Complete  field  frame.    Electro  Dynamic  Co. 


THE;  D.  c.  MOTOR 


accomplished  by  the  load-current,  varies  with  the  load,  as  it 
should.  The  flux  for  reversal,  being  thus  provided  exactly  where 
it  is  required,  the  shunt  fields  may  be  made  as  weak  as  is  desir- 
able to  secure  proper  speed  and  torque,  without  the  evil  effects 
of  armature  reaction.  The  brushes,  furthermore,  are  set  per- 


Wiring  Diagram,  showing  electrical  con- 
nections   between  the  armature, 
field,  and  "Inter-poles." 

Fig.  7°. 

manently  on  the  line  of  geometric  symmetry  between  the  poles, 
thus  enabling  the  machine  to  be  operated  in  either  direction. 
When  properly  adjusted,  it  is  found  that  the  angle  of  lag  usually 
given  to  the  brushes  of  shunt  motors  is  unnecessary.  This  motor 
also  takes  advantage  of  the  large  armature  and  commutator  to 


86  THE  MOTOR  AND  THE)  DYNAMO 

be  found  in  most  adjustable  speed  shunt  motors,  as  previously 
described,  thus  securing  a  range  of  speed  from  i  to  5  or  even 
i  to  6  in  either  direction,  without  sparking. 

In  connection  with  this  motor  it  should  be  noted  that  a  very 
slight  shifting  of  the  brushes  from  the  correct  position  causes  it 
to  behave  in  a  curious  way.  The  interpole  then  acts  somewhat 
like  a  differential  series  field,  causing  the  machine  to  speed  up 
till  the  c.  e.  m.  f .  is  above  the  line  voltage,  and  for  a  moment  the 
motor  acts  as  a  generator,  boosting  the  voltage  of  the  whole 
system  a  point  or  two.  This  immediately  causes  a  falling  off  in 
speed,  when  the  same  thing  is  repeated.  This  is  not  a  frequent 
phenomenon,  however,  where  these  motors  are  installed,  and  is 
guarded  against,  after  the  brushes  have  been  once  adjusted,  by 
fixing  them  in  position. 

All  means  of  speed  control  thus  far  considered  have  been  for 
increasing  the  r.  p.  m.  For  the  reverse  process,  when  the  full 
field  current  is  on,  there  is  no  convenient  method  adapted  to  con- 
stant speed  motors.  A  rheostat  in  series  with  the  armature, 
although  it  would  reduce  the  speed,  is  to  be  avoided  for  the 
reason  that  variable  load  makes  the  IR  drop  over  the  rheostat 
variable,  which  is  the  same  thing  as  applying  a  varying  voltage 
to  the  armature,  a  decreasing  voltage  with  increase  of  load.  The 
speed  current  curve  then  declines  toward  zero  speed. 

An  entirely  different  type  of  multi-speed  motors  to  those  thus 
far  considered  is  one  having  two  distinct  windings  on  the  arma- 
ture and  two  commutators.  By  having  the  numbers  of  inductors 
in  these  windings  related  as  2  to  3,  the  following  relative  speeds 
may  be  obtained : 

Connection.  Speed. 

2  and  3  opposed highest  speed,  5  X  a  constant 

2  alone 3  X  a  constant 

3  alone 2Xa  constant 

2  and  3  in  series lowest  speed,  i  X  a  constant 

A  similar  system  to  this  and  one  which  has  been  developed 
with  more  success  commercially  is  that  of  operating  motors  on 
multi-voltage  lines.  The  field  is  excited  on  the  highest  voltage, 
and  different  voltages  are  applied  to  the  armature,  according  to 


THE  D.   C.   MOTOR 


the  speed  desired.  The  various  voltages  are  obtained  by  dynamos 
operating  in  series.  Three  such  systems  are  in  existence,  and 
are  represented  in  Figs.  71,  72  and  73. 

Intermediate  speeds  are  obtainable  by  field  control.     All  these 
systems   differ   from   those   of   the   single   voltage   field   control 


0 

T 

61 

0 

T 

115 

1 

1*7 

I 

i. 

37 

0 

. 

Fig.  71.—  Ward 

Leonard. 

Speed 

ratios,  1:2:3:4:6:7. 

0 

T 

T 

/< 

,0 

I 

2.1-0 

0 

T 

10 

1 

100 

1  J 

Fig.  72. — Crocker  Wheeler.    Speed  ratios,  1:2:3:4:5:6. 


0 

T 

1 

0 

T            ,  J 

T       . 

0 

a. 

SO 

0 

ii 

i 

0 

0 

Fig-  73- — Bullock.    Speed  ratios,  3  :  4  :  5.5  :  7  :  9  5  :  12.5. 

method  in  the  fact  that  with  added  voltage  there  is  an  added  input 
and  added  horse-power  developed.  In  other  words,  these  latter 
systems  supply  constant  torque  with  various  speeds,  in  distinction 
to  constant  horse-power  with  various  speeds,  which  characterizes 
7 


88  THE  MOTOR  AND  THE  DYNAMO 

the  interpolar  motor  and  its  predecessors  in  the  market.     And 
when  it  is  considered  that  the  electric  motor  is  a  machine  which 

.Line 


Fig.  74- 


in   any    event   automatically    develops    torque    and    horse-power 
according  to  the  load  put  upon  it,  the  fact  that  the  interpolar 


THE  D.   C.   MOTOR 


90  THE:  MOTOR  AND  THE:  DYNAMO 

shunt  motor  operated  on  a  single  voltage  is  rapidly  superceding 
all  other  direct-current  devices  can  be  easily  accounted  for. 

(d)  Starting-boxes  and  Controllers. 

Together  with  the  adjustable  speed  motor  there  has  come  in 
a  new  form  of  starting-box  with  a  self-contained  field  rheostat. 
Fig.  74  shows  the  internal  connection,  and  Fig  75  the  external 
appearance  of  the  box.  The  handle  is  double.  The  movement 
for  starting  is  the  same  as  iA  the  ordinary  box.  At  the  end  of 


Front  view.  Back  view. 

Fig-  77-— Field  rheostat  for  generators.     G.  E.  Co. 


the  starting  stroke  the  handle  divides,  one  blade  being  held  by 
the  retaining  magnet,  the  other  being  movable  back  across  the 
box-face,  throwing  increasing  resistance  into  the  shunt-field  cir- 
cuit by  means  of  the  upper  row  of  studs. 

The  usual  form  of  field  rheostat,  used  with  the  ordinary  type 
of  starting-box,  is  shown  in  F'ig.  76.  Its  resistance  coils  or 
strips  are  embedded  in  porcelain.  The  type  of  rheostat  more 
particularly  used  for  voltage  control  in  generators  is  shown  in 
Fig.  77. 

In  regard  to  starting-boxes  for  shunt  and  compound  motors, 


THE:  D.  c.  MOTOR  91 

it  should  be  noted  that  the  resistance  coils  for  the  armature 
current  are  capable  of  carrying  that  current  only  for  a  short 
time  without  overheating.  The  box  should  therefore  never  be 
used  as  a  speed-reducing  rheostat,  unless  attached  to  a  motor 
much  smaller  than  that  for  which  it  was  designed.  Under  ordi- 
nary conditions  the  starting-box  for  shunt  motors  is  designed  to 
be  used  15  times  an  hour  without  overheating  and  it  is  calculated 
that  15  seconds  may  be  consumed  in  the  operation  of  starting. 


Fig.  78.— General  Electric  Co.  Ratchet-driven  remote  control  rheostat. 

Small  motors  and  motors  not  starting  under  load  require  much 
less  time  than  this. 

It  may  be  laid  down  as  a  general  rule,  that  a  motor  starting 
free  should  begin  to  rotate  when  the  starting  handle  is  on 
the  first  contact  point  of  the  box.  Under  load  the  handle  may 
be  moved  to  the  second  or  even  the  third  before  the  armature 
begins  to  rotate;  but  if  the  machine  does  not  then  start,  it  is  a 
sign  of  too  heavy  a  load  at  starting  or  of  some  other  trouble. 
Some  boxes  are  provided  with  an  overload  release,  which  is  in 


92  THE:  MOTOR  AND  THE:  DYNAMO 

effect  nothing  more  than  an  automatic  cut-out  or  circuit-breaker. 

For  stopping  a  shunt  or  compound  motor,  the  handle  of  the 
box  should  never  be  moved  back  across  the  studs,  as  this  will  burn 
and  roughen  them.  The  supply  switch  must  be  opened  instead, 
leaving  the  handle  to  return  automatically  to  its  first  posiion. 

For  the  series   motor  the  starting  device   is   simply   a   rheo- 


Starting  rheostat  for  series  motor. 


G.  K.  Co.  reversible  controller  for  series  motor. 
Fig-  79- 


stat  in  series  with  the  machine.  Because  of  the  heavy  current 
used  in  such  machines,  large  starting  torque  being  usually  sought, 
the  close-lying  knobs  and  light  coils  of  the  shunt  starting-box  are 
unsuited.  Fig.  79  represents  a  series  motor  controller  and  rheo- 
stat. The  contact  fingers  are  of  heavy  copper  and  spring-hinged. 
They  are  also  separated  from  one  another  by  thick  insulating 
partitions,  usually  of  asbestos.  An  electro-magnet  provides  flux 
for  blowing  out  the  arcs  at  contact  points. 


THE  D.   C.  MOTOR  93 

(e)  Motor  Uses. 

Because  of  the  great  convenience  and  other  advantages  of 
electric  driving  apparatus,  most  makers  of  machine  tools  and 
other  factory  appliances  to-day  equip  them  with  motors  and  pro- 
vide places  on  the  frames  for  installing  the  same.  In  their  new 
and  comprehensive  work  on  electric  motors,  Messrs.-  Crocker  and 
Arendt  enumerate  many  points  in  favor  of  separate  electric  drive 
for  the  machines  of  manufacturing  plants  as  against  the  older 
system  of  overhead  shafting  and  pulleys.  Some  of  these  are 
as  follows : 

(1)  The  lo<ss  of  power  incident  to  shafting  and  belts  is  pre- 
vented. 

(2)  Better  lighting  and  greater  cleanliness  are  obtainable. 

(3)  Floor  space  may  be  utilized  to  better  advantage,  it  being 
possible  to  place  a  machine  anywhere  and  to  face  it  in  any  di- 
rection. 

(4)  With  motors  of  wide  speed-range,  cone  pulleys  and  inter- 
changeable gear-wheels  become,  to  a  large  extent,  unnecessary. 

(5)  The  ease  and   quickness   of   speed   adjustment  not   only 
saves  the  time  of  operatives  in  the  shops,  but  by  encouraging  a 
greater  care  as  to  the  proper  speed  to  be  used,  insures  a  more  per- 
fect  product.      This   is   one    of   the   greatest   advantages.      See 
Fig.  80. 

(6)  Side-walls   and   roof-beams  may  be  of   lighter  construc- 
tion where  shafting  does  not  have  to  be  supported. 

(7)  In  cases  of  shut-downs,  part  of  a  plant  or  even  isolated 
and  widely  separated  machines  may  be  operated  without  the  loss 
of  power  incident  to  lines  of  shafting  and  pulleys. 

(8)  Individual  motors  draw  power  in  close  proportion  to  the 
work  they  are  doing. 

As  to  whether  original  cost  outweighs  these  advantages  is  a 
matter  that  must  be  decided  for  each  special  case. 

(f)  Traction  Motors. 

In  the  operation  of  series  motors  for  traction  purposes,  it  is  the 
custom  to  use  two  or  four  machines  to  a  car,  and  to  make 


94 


THE  MOTOR  AND  THE  DYNAMO 


the  one  machine  or  pair  serve  as  rheostat  to  the  other  machine 
or  pair  at  the  time  of  starting.    When  running  at  full  speed,  the 

/Vo./ 


ro 


L 


60 


//•  MINUTES 

CUTTING  SPEEDS  AND  TIME  REQUIRED  TO  FACE   A   72-INCH  CAST  IRON  DISK 
USING  THREE  STEPS  ON  THE  CONE   PULLEY. 

!  NO.  2 


CUTTING  SPCCD  IN  fr  PCR  Mi» 
§  3  S  § 

S\^ 

JO                    ZO                    3O                   40                 50 
7/MC  /N  MlNUTELS 

CUTTING  SPEEDS  AND  TIME  REQUIRED  TO  FACE  A  72-INCH  CAST  IRON  DISK 
WITH  LATHE  DRIVEN  BY  MOTOR  WITH  FIELD  CONTROL. 

Fig.  80.     By  courtesy  of  the  G.  E.  Co. 

machines  are  operated  in  parallel.  Besides,  a  rheostat  in  series  with 
each  machine  provides  the  intermediate  steps.  The  transition  from 
series  to  parallel  connection  is  an  operation  of  some  degree  of 


THE  D.   C.   MOTOR 


95 


complication.  Two  types  of  series-parallel  hand  controllers  are  in 
most  general  use.  Type  K  shunts  and  short-circuits  one  of  the 
motors  when  changing  from  series  to  parallel  connection.  Type 
L  controller  opens  the  power  circuit  in  making  the  change.  The 
series  of  steps  in  the  first  type  is  illustrated  in  Fig.  Si.  It  will 


#3      *4  Motor  Motor 


& 


4  r-J  tRjuTjiriJiriJ — °-^ — CK/W 

TJ  LnArulnjUL — ow — ow- 
r-J  LnAjiruinjb — OWY— ov\^< 


-CKAA  G 


K>V^ 


dllmnJlTLJll — ow^ 


CONNECTIONS  FOR  SMALL  CONTROLLERS 
Fig.  81. 

be  observed  that  not  every  point  is  a  running  position.  The  rheo- 
stat is  not  heavy  enough  to  stand  the  operating  current  for  any 
length  of  time,  and  some  points  are  passed  over  without  being 
indicated  either  in  the  motion  or  by  marks  on  the  top  of  the 
controller  box. 

These  controllers  and  the  motors  operate  on  voltages   rang- 


THE:  MOTOR  AND  THE;  DYNAMO 


ing  from  500  to  600  and  the  horse-power  of  the  motors  ranges 
from  25  to  50.  Above  this  size  the  multiple  unit  system  of  con- 
trol is  preferred.  This  system,  used  in  electric  trains,  consists 
of  a  master  controller  drawing  but  a  small  current  and  operated 
in  any  car  of  the  train  and  the  larger  motor  controllers  carried 


T-nWi^^ 


CONNECTIONS  FOR  LARGE  CONTROLLERS 
Fig.  82. 

with  the  resistances  under  the  car  and  operated  either  by  sol- 
enoid coils  or  by  compressed  air,  in  unison  with  the  movements 
of  the  master  controller.  In  changing  from  series  to  parallel 
connection,  the  Sprague  General  Electric  automatic  control  sys- 
tem provides  means  of  keeping  both  motors  in  operation  and  pre- 
serving their  torque  throughout  the  change.  (The  same  is  true 


THE:  D.  c.  MOTOR 


97 


of  the  new  larger  type  K  controllers.  See  Fig.  82).  Further- 
more it  sets  a  maximum  limit  relay  to  the  rate  of  motion  of  the 
controllers  and  so  to  the  acceleration  of  the  train.  For  fuller 
acounts  of  these  interesting  controlling  devices,  the  reader  is 
referred  to  works  on  electrical  railway  engineering,  such  as 
Sheldon  and  Haussman  Electrical  Railways,  published  by  Van- 
Nostrand  Co.,  1911. 

(g)  The  Motor-Dynamo. 

This  is  the  proper  point  at  which  to  introduce  the  motor-dyna- 


Fig.  83.— Motor-dynamo.    G.  E.  Co. 

mo,  a  machine  having  two  distinct  armature  windings  on  the 
same  core  or  separate  cores  with  a  commutator  at  either  end.  It 
may  be  used  to  step  direct-current  voltages  up  or  down  by  a 
given  ratio,  according  to  the  relative  number  of  inductors  in  the 
two  windings.  The  chief  use  of  this  instrument  is  as  a  balancer 
in  the  Edison  three-wire  system.  In  such  case  the  two  windings 
and  voltages  are  alike.  The  modern  type  of  this  machine  is 
double  in  field  and  armature.  (Fig.  83.) 


THE  MOTOR  AND  THE)  DYNAMO 


The  balancer  is  employed  when  it  is  desired  to  run  a  three- 
wire  system  from  a  single  22O-volt  generator.  Fig.  84  will  make 
the  operation  clear.  As  long  as  the  system  is  perfectly  balanced, 


Fig.  84. — Three-wire  S3rstem  with  balancer. 

the  balancer  has  nothing  to  do.  But  in  the  case  in  the  figure, 
the  return  of  5  amperes  on  the  middle  wire  divides,  about  2^2 
amperes  operating  the  balancer  as  a  motor  by  means  of  one  of 
the  armature  windings  and  causing  the  other  winding  to  gene- 
rate the  extra  2.y2  amperes  required  in  the  positive  wire  of  the 
circuit. 

(h)  Losses  of  Power  in  Generators  and  Motors. 

In  direct-current  machines  the  losses  are  usually  divided  as  in 
the  following  table. 

c  (a)  Watts  lost  in  shunt  field,  I/R/. 
Copper  losses  ]  (b)  Watts  lost  in  series  field,  I/R,. 

(  (c)  Watts  lost  in  armature,  Ia2R«. 

f  (d)  Eddy  current  losses  in  armature  iron 
and  pole-faces,  varying  approximate- 
ly as  the  square  of  the  speed. 

I  (e)    Hysteresis   losses   in    armature    core, 
Stray  power  losses  <;  .  ,       .       _.,  fi 

varying  as  speed  and  as  B1'6. 

I  (f)  Bearing  friction,  brush  friction  and 
windage  varying  approximately  as 
the  speed. 

A  reference  to  page  n,  Chapter  II,  is  all  that  is  necessary  to 
make  the  copper  losses  clear.  They  vary  with  the  square  of  the 
current  and  hence  depend  upon  the  load.  Eddy-currents  occur 
whenever  solid  masses  of  conducting  material  move  rapidly 


THE  D.   C.   MOTOR  99 

through  an  un-uniform  magnetic  field.  The  armature-core, 
though  laminated,  is  not  wholly  free  from  eddy-currents.  Again, 
the  flux  in  the  air-gap  between  field  and  armature  is  really  of  the 
shape  shown  in  Fig.  85. 

The  shifting  of  these  tufts  over  the  pole-face  engenders  in  it 
an  e.  m.  f.  and  hence  electric  currents  in  the  form  of  little 
whirls.  A  similar  thing  occurs  in  the  armature  conductors  them- 
selves, especially  if  they  have  considerable  superficial  area. 
Since  e.  m.  f.  and  hence  current  varies  as  rate  of  cutting  the 
flux,  and  watts  vary  as  the  square  of  the  current,  these  losses 
vary  as  the  square  of  the  speed. 

Hysteresis    results    from   the    reversals   of   magnetism   in   the 


Fig.  85. 

armature  core.  Once  in  every  revolution  of  a  bi-polar  machine, 
the  armature  iron  goes  through  a  complete  magnetic  cycle.  The 
watts  lost  depend  upon  the  degree  of  saturation  and  the  fre- 
quency. The  load  carried  by  the  machine  has  little  influence 
on  the  eddy  current  and  hysteresis  losses,  the  only  effect  of 
the  armature  current  in  this  direction  being  its  reaction  on 
the  field  flux. 

With  this  exception  and  the  fact  that  during  operation  the 
tension  on  the  belt  may  increase  the  bearing  friction,  all  the 
stray  power  losses  of  the  dynamo  machine  are  the  same  when 
the  machine  is  running  free  as  when  it  is  loaded,  provided  the 
field  excitation  and  the  speed  are  the  same  as  when  under  load. 
The  stray  power  test  of  efficiency  consists  in  determining  the 


100 


THE:  MOTOR  AND  THE:  DYNAMO 


dynamo  losses,  when  the  machine  is  thus  running  light.     Then 
for  generators 

~  .  output 

Per  cent,  efficiency  =  -  X   100. 

output  -}-  losses 

and  for  motors 

„  .  imput  —  losses 

Per  cent,  efficiency  =  -     — : —  -  X   100. 

input 

In  any  machine  the  losses  are  the  same,  whether  operating  as 
a  generator  or  as  a  motor. 

The  citation  of  an  actual  case  will  make  this  clear.  Connec- 
tions for  the  test  are  shown  in  Fig.  86.  Suppose  a  certain  motor, 
when  loaded  and  operating  on  a  circuit  of  115  volts,  to  draw  31 
amperes  total  current,  the  speed  being  1,200  revolutions  per 


Fieldl   RKeo^tat 


Fig.  86. — Connections  for  stray-power  test. 

minute.  If  we  know  the  field  current  at  this  time,  say  I  ampere, 
the  working  conditions  of  field  and  speed  can  be  readily  repro- 
duced and  the  stray  power  losses  determined  as  follows  :  Operate 
the  motor  free  from  load,  and  by  means  of  the  field  rheostat  and 
ammeter  reproduce  the  field  current  of  i  ampere  as  closely  as 
possible.  Next  by  means  of  a  rheostat  in  the  armature  circuit 
cut  down  the  speed  to  the  load  speed  value,  namely,  1,200  revolu- 
tions per  minute,  and  read  the  amperes  furnished  to  the  armature 
and  the  voltage  between  the  brushes.  Suppose  these  to  be  2 
amperes  and  75  volts.  The  watts  furnished  to  the  armature  are 
then  =.  2  X  75  =  150.  Of  these  there  are  consumed  in  the  arma- 
ture resistance  I«2Ra  •=  22  X  0.5  =  2  watts,  leaving  148  watts 
This  is  known  as  stray  power,  because  together  with  the  I/R/  of 


THE  D.   C.   MOTOR 


IOI 


Fig.  87.— Mill  type  motor.    G.  E.  Co. 


Fig.  88.— Motor-operated  crane.    Crocker-Wheeler  Co. 


IO2 


THE)   MOTOR  AND  THE  DYNAMO 


the  field  it  represents  the  power  used  to  drive  the  machine  at  load 
speed  doing  no  work  whatsoever  but  to  overcome  the  opposing 
forces  of  the  machine  itself.  The  total  losses  of  the  machine, 
then,  when  loaded  to  the  extent  named,  are  this  stray  power  loss 


Fig.  89.— Motor-operated  press. 


plus  the  copper  losses  of  field  I/R/-  and  armature 


latter  are,  namely,  I/E/-,   or   i    X 


la.     These 
115  =   115  watts  in  the  field 


and  3<D2  X  0.5  =  450  watts  in  the  armature.    The  total  loss  under 
this  condition  of  load  is  therefore  115  +  450  +  148  =  713  watts. 


THE  D.   C.   MOTOR  103 

Now  the  input  was  IE  =31  X  115  =  3.565  watts.  The  output 
must  therefore  be  input  minus  losses,  or  3,565  —  713  =  2,852 
watts,  and  the  efficiency  of  the  motor  for  this  load  and  field 

excitation  must  be—1— — -  X  100  —  80  (per  cent.). 

The  stray  power  losses  determined  in  this  way  on  a  shunt 
motor  are  fairly  constant  throughout  a  considerable  range  of 
speeds  and  loads.  The  test  may  also  be  made  on  a  series  or  a 


Fig.  90.  -Motor-operated  lathe.     Reliance  Electric  &  Engineering  Co. 

compound  motor,  but  rheostats  of  large  carrying  capacity,  such  as 
banks  of  incandescent  lamps  or  a  water-barrel  rheostat,  must  be 
used.  In  testing  a  shunt  dynamo  machine,  however,  this  method 
is  most  convenient,  since  it  draws  but  little  power  even  for  very 
large  machines.  The  readings  of  voltage,  field  current,  load  cur- 
rent, and  speed,  must  previously  have  been  taken  under  conditions 
of  actual  operation.  This  applies  equally  to  testing  the  generator 
and  the  motor. 

The    sitray-power    method    of    obtaining    efficiency   gives    re- 
sults a  little   too  high,   owing  to  the   fact  that  all   defects   in- 
8 


IO4 


THE  MOTOR  AND  THE  DYNAMO 


cident  to  full  load  current  in  the  armature  are  lacking.  The 
error  is  however  slight,  and  the  results  are  likely  to  be  more 
accurate  than  might  be  determined  by  a  clumsy  or  imperfect 
friction  brake.  In  cases  of  too  great  discrepancy  because  of 
small  load  current,  a  machine  may  be  tested  by  what  is  known  as 


Fig.  91.— Motor-operated  milling  machine.     Reliance  Electric  &  Engineering  Co. 

the  pumping-back  test.     In  this  there  are  two  machines  exactly 
alike  coupled  in  series,  one  furnishing  current  to  the  other.  Pre- 
serving the  load  current  in  this  way  through  the  armatures,  the 
test  proceeds  similarly  to  the  stray  power  test. 
Figs.  87,  88,  89,  90  and  91  show  a  number  of  motor  applications. 


CHAPTER  VI. 


THE  ALTERNATING  CURRENT  AND  ITS  MEASUREMENT. 


(a)  The  A.  C.  Wave. 

An  alternating  current  is  one  which  periodically  reverses  its 
direction  of  flow.  The  alternating  currents  of  commerce  are 
restricted  to  a  certain  number  of  reversals  per  second  and 
approximate  a  particular  ideal  wave-shape,  known  as  a  sinusoidal 
curve.  The  following  will  make  this  clear: 

The  Sinusoidal  Curve. — Consider  the  point  P  (Fig.  92)  to  be 
moving  uniformly  around  the  circumference  of  a  circle,  or  along 
the  path  a  b  c  d  a.  The  projection  of  this  motion  on  a  vertical 


' 


v      _y 


Fig.  92. —The  sinusoidal  curve. 

diameter  through  o  becomes  the  motion  o  b  d  o.  This  latter  is 
known  as  simple  harmonic  motion,  and  the  circle  corresponding 
is  called  the  circle  of  reference.  As  the  radius-vector  joining 
o  and  P  sweeps  around  the  circle,  the  angular  displacement 
(denoted  by  6)  of  P  from  a  passes  through  all  values  from 
zero  to  360°.  The  corresponding  linear  displacement  from  o 
of  the  projection  of  P  on  the  vertical  diameter  is  equal  at  any 
instant  to  the  radius  (r)  X  sin  0,  and  is  known  as  a  harmonically 
varying  quantity.  If  we  draw  a  horizontal  line  ax  divided  to 
represent  degrees,  and  from  this  up  and  down  lay  off  the  values 
of  the  corresponding  linear  displacements  and  join  these  points, 
we  shall  have  a  so-called  sinusoidal  curve,  or  curve  of  sines; 
viz.,  a  b'  c'  d'  x.  A  curve  of  cosines  would  have  the  same  shape, 
and  would  differ  only  in  position,  being  90°  removed  along  the 


io6 


THE   MOTOR  AND  THE  DYNAMO 


axis  ax.     Cos  0  —  sin  (0  -\-  90).     The  dotted  line  represents 
the  curve  of  cosines. 

In  order  to  explain  why  the  shape  of  the  alternating-current 
wave  approximates  to  such  a  curve,  it  is  necessary  to  show  that 
the  rate  of  change  of  a  harmonically  varying  quantity,  like  the 
sine  of  the  varying  angle  6,  is  another  harmonically  varying  quan- 
tity, such  as  the  cos  0.  This  is  expressed  at  once  by  the  differ- 
ds'm  0 


ential  calcus  as 


dt 


=  cos  0.     It  can  be    observed    also    in 


the  curves.  The  increase  in  the  sine  values  is  greatest  at  a  and 
gradually  becomes  less  and  less  till  at  b'  variation  is  zero.  This 
variation  is  expressed  by  the  first  quarter  of  the  declining  dotted 
curve,  which  crosses  the  axis  at  the  90°  point.  From  b'  to  c'  the 
sine  value  decreases,  at  first  slowly  and  finally  with  greatest 
rapidity  at  c' .  This  rate  of  change  is  expressed  by  the  second 


Fig.  93- 

quarter  of  the  dotted  curve,  whose  greatest  negative  value  is 
opposite  c'.  And  so  on. 

Now  let  the  horiontal  lines  in  Fig.  93  represent  a  uniform 
magnetic  field  of  a  two-pole  generator,  and  let  P  be  the  end 
view  of  an  inductor  moving  around  with  the  rotating  armature. 
The  rate  of  cutting  these  lines  of  force  is  the  e.  m.  f.  generated 
between  the  terminals  of  the  inductor  in  absolute  units  (see  p.  5, 
Chap.  II). 

From  a  comparison  of  this  figure  with  Fig.  92,  it  will  be 
seen  that  this  rate  of  cutting  is  the  rate  of  change  of  the  sine 
of  the  varying  angle  0.  This  is,  as  has  been  shown,  a  harmon- 
ically varying  quantity.  Hence  the  generated  e.  m.  f.,  which  in 


ALTERNATING  CURRENT  AND  ITS   MEASUREMENT  IO7 

the  absence  of  a  commentator  is  the  alternating-current  e.  m.  f .', 
can  be  represented  by  the  curve  of  cosines, — or  equally  well,  as 
to  shape,  by  the  curve  of  sines. 

In  fact,  if  a  be  the  starting  point  as  in  Fig  93,  we  start  with 
the  maximum  value  of  the  e.  m.  f.  generated.  A  more  appropriate 
point  from  which  to  measure  the  angles  of  rotation  would  be 
where  the  e.  m.  f.  is  at  its  lowest  value  or  zero,  the  lines 
of  force  being  cut  with  least  rapidity  at  this  point.  The  curve 
would  thus  be  removed  90  degrees  in  advance  of  the  cosine 
curve,  and  would  be  the  true  sine  curve.  If  the  maximum  value 
attained  by  the  e.  m.  f.  during  the  cycle  be  expressed  by  the 
radius,  or  by  sin  90  degrees,  then  the  value  of  e.  m.  f .  at  any 
moment  of  time  t,  dating  from  the  passage  of  this  origin  by  the 
coil  would  be  the  maximum  value  of  the  e.  m.  f .  X  sin  6.  De- 
noting angular  velocity  by  w,  any  angle  0  so  measeared  would 
be  w/. 

Hence  <?instan.  =  Emax  sin  w/. 

This  is  made  more  clear  by  the  so-called  clock  diagram.  In 
Fig.  94  let  the  length  of  radius  vector  o  E  represent  Emax. 


Fig.  94. — Clock  diagram. 

The  current  may  be  similarly  expressed.  Because,  however, 
of  self-induction,  which  is  almost  always  present  in  alternating- 
current  circuits,  the  current  curve  seldom  coincides  with  the 
e.  m.  f .  curve,  as  will  be  explained  later,  and 

Install.    ==  Imax  sin   ((•)/ <&) 

where  3>  is  the  angle  by  which  the  current  fmstan.  lags  behind 
the  voltage.  A  lagging  current  would  be  indicated  by  Fig.  95, 
the  e.  m.  f .  reaching  its  maximum  value  while  the  current  is  still 


THE;  MOTOR  AND  THE  DYNAMO 


on  the  increase.     The  angle  $  expresses  the  difference  in  phase 
between  the  two. 

The  number  of  cycles  (  <s*  )  per  second  is  known  as  the  fre- 
quency. There  are  twice  as  many  alternations  as  there  are  cycles. 
In  a  two-pole  generator,  there  is  one  cycle  per  revolution.  The 


FiK-  95-— Lagging  current. 

frequency  may  therefore  be  found  for  any  machine  by  multiply- 
ing revolutions  per  second  by  the  number  of  pairs  of  field-poles. 

(b)  Mean,  Average  and  Effective  Values. 

An  alternating  current  of  any  definite  number  of  amperes 
means  a  current  that  will  have  the  same  heating  effect  as  that 
number  of  direct-current  amperes.  The  hot-wire  ammeter  was 
one  of  the  first  forms  of  meter  used  for  measuring  alternating 
currents.  The  formula  for  the  calories  (H)  developed  by  any 
current  is 

H  =  0.24  PRf. 

Thus  it  comes  about  that  the  effective  amperes  alternating- 
current  are  not  the  maximum  amperes  expressed  by  the  peak  of 
the  wave,  nor  the  mean  between  this  maximum  and  zero,  nor 
even  the  average  value  of  all  the  instantaneous  amperes  of  a 
complete  cycle,  but  rather  the  square  root  of  the  average  square 
of  the  instantaneous  amperes. 

In  the  direct-current  ammeter  with  fixed  permanent  magnet 
and  movable  coil,  the  usual  type,  the  pointer  attached  to  the 
coil  moves  across  a  scale  of  even  divisions.  Such  an  instru- 
ment would  not  register  alternating-current,  except  by  a  possible 
trembling  motion  of  the  pointer.  In  the  alternating-current 
instrument,  the  magnet  is  replaced  by  a  coil,  the  movable  coil 
turning  in  the  flux  set  up  by  this  fixed  coil.  As  the  current  al- 


ALTERNATING  CURRENT  AND  ITS   MEASUREMENT 


I09 


ternates  simultaneously  in  the  two  coils,  the  deflection  is  in  one 
direction  only.  But  this  deflection  is  now  necessarily  propor- 
tional to  the  square  of  the  current,  and  the  scale  divisions  are 
uneven.  The  pointer  does  not  oscillate,  but  because  of  the  in- 
ertia of  the  moving  element  it  takes  up  a  definite  position.  In  this 
instrument  too,  therefore,  as  well  as  in  the  hot-wire  ammeter, 
the  deflection  is  that  caused  by  the  average  square  of  all  the 
instantaneous  current  values  of  the  complete  cycle  and  the  am- 
peres marked  on  this  scale  are  the  square  root  of  this  mean  or 
average  square.  Thus  it  is  that  this  value  rather  than  any  other 
value  comes  to  be  regarded  as  the  direct-current  equivalent  or 
the  effective  alternating-current  amperes.  The  same  is  true  of 
alternating-current  volts. 

Now  the  average  value  of  the  sines  for  a  complete  cycle  of 
360  degrees  equals  the  average  value  of  the  cosines,  and  also 

Average  sin2  to/  =  average  cos2  to/. 
But      sin2  to/  -}-  cos2  to/  =  i       for  all  values  of  to/. 
Hence       average  sin2  to/  ==  */£,and  ^/average  sin2  ut   =  ily'^T. 

Since         ^instan.   =  Emax  sin  to/, 

average  <?2instan.  ==  E2max  X  average  sin2  to/  =  tf  E2max, 

T? 

and          Ecffective  —  1/av.  <?'2instau.     — 


V    2 


=  0.707  K, 


The  same  may  be  deduced  practically  from  the  following  table : 


Angle 

Sine 

Sine  squared 

O 

o.ooo 

o.oo 

30 
60 
90 
120 

150 

0.500 
0.866 

I.OOO 

0.866 
0.500 

0.25 
0-75 

1.  00 

0.75 
0.25 

6)3.732 

6)3.00 

Average  sin  =       0.622 


Av.  sin2  (at  =     0.5 
1  '05"  =       0.707 


IIO  THE   MOTOR  AND  THE  DYNAMO 

(c)    Inductance  or  Self-induction. 

Because  of  the  magnetic  flux  which  surrounds  a  current-bear- 
ing wire,  any  change  of  current  is  accompanied  by  a  change  of 
flux.  By  Lenz's  law,  this  change  of  flux  due  to  change  of  cur- 
rent in  the  wire  tends  to  set  up  an  e.  m.  f.  in  a  direction  such 
as  to  oppose  the  change  of  current.  Thus  an  electric  current 
has  a  property  very  similar  to  the  inertia  of  a  moving  mass. 
Like  inertia,  this  is  a  property  of  the  type,  shape,  and  dimensions 
of  the  circuit  and  is  independent  of  the  current  in  it.  A  helix 
has  more  self-induction  than  a  straight  wire,  and  a  helix  con- 
taining an  iron  core  has  more  than  one  without. 

Inductance  (L)  is  measured  in  henries.  A  henry  is  such  an 
inductance  as  will  cause  a  counter  e.  m.  f .  of  one  volt,  when  the 
current  changes  at  the  rate  of  one  ampere  per  second.  This 
c.  e.  m.  f .  is  of  the  nature  of  an  ohmic  resistance,  is  measured  in 
ohms  and  is  called  reactance.  Since  angular  speed  per  second  in 
the  cycle  is  denoted  by  to,  or  iirf,  the  value  of  this  reactance  is 
27T/L,  where/ is  the  frequency. 

The  calculus  expresses  this  as  follows:  The  rate  of  change 
of  current  is  —  ;  and  since  i  (instantaneous)  —  Imax  sin  W, 

— —  —  wlmax  cos  (at  =  wlmax  sin   (W -}-    90°).     This  is  counter 

e.  m.  f.,  and  the  effective  value  thereof  is  wleff  (or  27r/~Ieff)  for 
each  henry  of  the  circuit  and  is  90  degrees  removed  from  the 
current  causing  it.  This  being  a  c.  e.  m.  f.,  the  effective  volts 
applied  to  overcome  it  and  cause  the  current  to  flow  must  be 
1 80  degrees  removed  in  phase,  or  90  degrees  from  the  current 
on  the  opposite  side.  By  clock  diagram  we  have  Fig.  96.  IR 
is  plotted  in  the  direction  of  the  current,  they  being  both  in  the 
same  phase.  2?r/LI  is  plotted  90  degrees  in  advance  of  the  cur- 
rent, being  the  impressed  volts  which  at  this  frequency  f  is  re- 
quired to  force  current  I  through  the  circuit  of  L  henries  in- 
ductance (that  is,  through  2?r/L  ohms  reactance).  The  resultant 
of  these  two  e.  m.  f.'s  gives  the  effective  e.  m.  f.  for  this  circuit, 


AI/TERNATING  CURRENT  AND  ITS   MEASUREMENT 


III 


namely  OE,  whose  direction  shows  the  phase  relation  between 
current  and  voltage  in  this  case. 

All  this  can  be  made  clear  diagrammatically  as  follows :     In 
Fig.  97  the  current  curve  abed  changes  most  rapidly  at  a  and  c, 


RI 

Fig.  96. 

hence  the  c.  e.  m.  f .  curve  is  greatest  at  these  two  points,  being 
negative  where  the  current  is  positive,  and  is  represented  by  the 
curve  cfgh.  To  oppose  this  c.  e.  m.  f.,  the  impressed  volts  which 
cause  the  current  to  flow  must  be  represented  by  the  cruve  kflh, 
which  precedes  the  current  curve  abed  by  90  degrees. 


Fig.  97- 


The  vector  sum  of  resistance  and  reactance  is  called  imped- 
ance. The  reciprocal  of  impedance  is  called  admittance,  sim- 
ilarly as  the  reciprocal  of  resistance  in  direct-current  is  called 
conductance.  Impedance,  like  reactance  is  measured  in  ohms. 


112  THE  MOTOR  AND  THE)  DYNAMO 

(d)  Capacity  in  Circuit. 

When  an  alternating  e.  m.  f.  is  applied  to  the  terminals  of  a 
condenser,  the  latter  is  charged  and  discharged  in  rapid  succes- 
sion, each  plate  receiving  alternately  a  -|-  and  --  charge.  The 
effect  is  the  same  as  if  the  alternating-current  went  through  the 
condenser,  which  offers  a  resistance  effect  to  the  flow  of  cur- 
rent,— also  a  form  of  reactance.  But  whereas  inductance  causes 
a  lagging  current,  a  condenser,  or  capacity,  in  the  circuit  causes 
the  current  to  precede  or  lead  the  e.  m.  f.  in  phase. 

The  calculus  explains  this  as  follows :  If  K  is  capacity  in 
farads,  E  pressure  in  volts,  and  Q  quantity  of  electricity  in 
coulombs  or  ampere-seconds,  then 

Q  =  KE 

It  must  be  remembered  that  a  farad  is  that  capacity  which 
will  receive  a  charging  current  of  one  ampere  when  the  e.  m.  f. 
is  changing  at  the  rate  of  one  volt  per  second. 

The  condenser  current  4  at  each  instant  is  proportional  to  the 
rate  of  change  of  pressure,  or 


But  ^instan.   ==  Emax  Sin 

de_ 
dt 


and  —'-   =  (O  Emax  COS 


Hence  -~  =  Emax  cos  to/  =  Emax  sin  (o>/  -J-  90°)  =  e^  where 
o>K 

Ck  is  the  instantaneous  pressure  in  phase  with  the  condenser  cur- 
rent. Hence  the  effective  value  of  Ik  divided  by  o>K  (or  by  27T/K) 
is  the  value  of  the  effective  e.  m.  f .  causing  this  current  Ik  and 
is  90°  behind  I  in  phase. 

In  Fig.  97  let  efgh  represent  the  e.  m.  f.  wave.  At  e  the 
e.  m.  f.  is  changing  least  rapidly,  and  so  the  current  flowing  into 
the  condenser  is  least,  giving  a  as  the  current  point.  At  / 
the  e.  m.  f .  is  changing  most  rapidly,  hence  current  value  is  high- 
est, or  at  b.  From  /  to  g  the  e.  m.  f .  is  still  increasing,  but  less 
rapidly,  hence  the  current,  although  still  positive,  decreases,  and 
so  on. 


ALTERNATING  CURRENT  AND  ITS  MEASUREMENT 


In  the  vector   diagram, 


I 


27T/K 


would   be   plotted   therefore 


1  80  degrees  removed  from  2?r/LI,  or  in  the  opposite  direction. 
Hence  in  the  vector  diagram  Fig.  96,  it  should  be  laid  off  down- 
ward. The  complete  effect  of  resistance,  inductance  and  capacity 
in  a  circuit  would  be  represented  vectorially  as  in  Fig.  98. 

,   the  reactance  due  to  capacity  and   27T/L,,   the  react- 


ance   due   to    self-induction,   must   be   added    algebraically.      In 
this  case  27T/L  is  the  larger.    The  vector  sum  of  these  reactances 


-ir 


(X)    and  the  resistance    (R)   of  the  circuit  is   represented  by 
the  hypothenuse  of  the  triangle  and  is  the  impedance  (Z)  of  the 


circuit.     Hence  Z  = 


It  is  seldom  possible  to  have  resistance  in  an  alternating-cur- 
rent circuit  without  inductance  and  vice-versa.  Let  there  be,  how- 
ever, an  ideal  circuit  composed  of  pure  resistance,  pure  induc- 
tance and  pure  capacity,  connected  in  series,  and  let  the  drop  over 
each  part  of  the  circuit  be  obtained  separately  by  a  voltmeter, 
the  current  in  the  circuit  being  maintained  constant,  then  the 
vector-sum  of  the  resistance  drop  and  reactance  drop  will  be  the 
impedance  crop,  or  difference  of  potential  across  the  circuit  as  in 
Fig.  99. 


114  THE   MOTOR   AND  THE  DYNAMO 

(e)  Power  in  A.  C.  Circuits. 

From  the  fact  that  the  voltage  and  current  are  seldom  in 
phase  in  alternating-current  circuits,  it  is  at  once  apparent  that 
the  volt  amperes  is  usually  larger  than  the  watts.  The  ratio 
of  these  two  quantities  is  known  as  the  power  factor,  and  is  the 
cosine  of  the  angle  (f>  on  the  vector  diagrams  Figs.  96  and  99. 

Treating  Fig.  99  as  a  clock  diagram,  counter-clockwise  rota- 
tion being  regarded  as  positive,  let  the  volts  be  plotecl  in  the 
direction  OE.  The  current  will  then  be  in  the  direction  OI, 
lagging  behind  the  volts  by  the  angle  <f>,  due  to  impedance.  This 
direction  will  correspond  to  that  of  the  resistance  drop,  since 


I 


T-  99- 


pure  resistance  causes  no  change  of  phase.     The  component  of 
OE  in  the  direction  OI  is  then  OE  cos  <£  and 
watts 


volts  X  amps. 


—  cos  <£  =  power  factor. 


We  thus  see  that  the  impressed  volts  in  an  alternating-current 
circuit  are  made  up  of  two  components,  one  overcoming  the  resist- 
ance and  the  other  the  reactance.  It  is  sometimes  of  advantage 
to  interchange  OE  and  OI  on  the  diagram,  considering  clockwise 
rotation  as  positive.  Then  one  may  speak  of  two  components  of 
current,  the  power  component,  which  is  in  the  direction  of  IR, 
and  the  wattless  component  at  right  angles  to  it. 


ALTERNATING  CURRENT  AND  ITS  MEASUREMENT  115 

The  formula  may  be  deduced  mathematically,  as  follows : 
e  =  E  sin  <ot 
i  =  I  sin  (a>/  —  <£) 
ei  =  El  sin  w/  sin  (W  —  <£) 
sin  (w/  —  <£)  =  sin  <*>/  cos  <£  —  cos  w/  sin  <£ 
«  =  El  sin2  to/  cos  <£  —  El  cos  W/  sin  to/  sin  <£. 

Average  «  =  El  cos  <£  av.  sin2  to/  —  El  sin  <£  av.  (cos  to/  sin  to/) . 

But      av.  sin2  o>/  =  ^    and  av.  sin  to/  cos  to/  =  o 

hence,        av.  «  =  watts  =  -    -  cos  <£ 

which  is  the  same  as  -  cos  <£  or  Eeff  Ieff  cos  <#>. 

1/2          1/2 

This  cos  <£  is  again  the  power  factor,  or  that  quantity  by 
which  the  effective  volt  amperes  must  be  multiplied  to  obtain 
effective  watts. 

Since  watts  is  a  rate  of  working,  it  may  be  represented  by  an 
area  formed  by  the  products  of  volts  by  amperes  at  each  succes- 
sive instant  of  time.  Those  areas  which  lie  above  the  line, 
being  due  to  the  product  of  quantities  of  like  sign,  are  posi- 
tive and  denote  power  furnished  by  the  circuit.  Those  which 
lie  below  the  line  are  negative  and  represent  power  withdrawn 
by  or  used  up  in  the  circuit.  See  Figs.  100,  101  and  102. 

(f)  Alternating  Current  Measuring  Instruments. 

It  will  be  remembered  that  the  most  common  type  of  direct- 
current  ammeter  or  voltmeter  consists  essentially  of  a  movable 
coil  operating  in  the  field  created  by  a  permanent  magnet.  Ob- 
viously with  an  alternating  current  in  the  coil  no  steady  position 
of  the  needle  could  be  maintained.  In  fact,  unless  the  alterna- 
tions were  very  slow  or  the  movable  parts  very  free  from  in- 
ertia, the  needle  would  only  tremble  slightly  back  and  forth  or 
would  refuse  to  move  at  all.  This  difficulty  is  avoided  by  elimi- 
nating iron  from  the  instrument  and  allowing  a  stationary  coil 
bearing  the  current  to  furnish  an  alternating,  that  is,  continually 
reversing,  magnetic  field.  The  movable  coil  is  supplied  from  the 
same  source  and  the  current  in  it  alternates  at  the  same  rate  and 


n6 


MOTOR  AND  THE  DYNAMO 


TTS 


Fig.  100. — Power  in  a  non-inductive  circuit. 


Fig.  ioi.— Current  lagging  90°.    Algebraic  sum  =  zero.    So-called  wattless  current. 


ALTERNATING  CURRENT  AND  ITS   MEASUREMENT 


117 


in   synchronism   with  the  magnetic   field.     This   creates  a   uni- 
directional torque  on  the  movable  coil. 

In  the  case  of  the  wattmeter  as  well  there  are  two  coils, 
one  movable  and  the  other  stationary.  The  one,  however,  is  in 
series  with  the  circuit,  is  of  low  resistance,  and  bears  the  current 
(amperes).  The  other  is  of  high  resistance  and  voltmeter-like 
is  tapped  across  the  circuit.  The  combined  effect,  therefore,  is 
at  each  instant  proportioned  to  the  product  of  the  volts  by  the 


Fig.  102.— Usual  lagging  current.     Power- factor  less  than  i. 

amperes.  With  such  instruments  there  is  necessarily  a  power- 
factor  of  the  wattmeter  itself  which  varies  with  the  character 
of  the  circuit.  It  is,  however,  usually  small. 

An  instrument  having  an  extremely  light  movable  part  has 
in  recent  years  been  perfected,  known  as  the  oscillograph.  In 
this  instrument,  by  means  of  a  mirror  oscillating  with  the  cur- 
rent, a  photographic  record  may  be  taken  of  the  current  curve, 
and  simultaneously  on  the  same  film  by  means  of  other  mirrors, 
each  on  its  own  motive  device,  the  e.  m.  f .  wave  or  any  other  de- 


nS 


THE  MOTOR  AND  THE)  DYNAMO 


sired  may  be  photographed.    Fig.  103  is  a  reproduction  of  such  a 
photograph. 

(g)  Voltage  in  A.  C.  Circuits  in  Series. 

It  is  possible  to  have  a  non-inductive  circuit,  that  is  a  pure 
resistance,   but   the   reverse,   a   pure   inductance   without    resist- 


Fig.  103.— Sinusoidal  curves.    Courtesy  of  G.  F,.  Co. 

ance  is  of  course  impossible.  The  theoretical  diagrams,  there- 
fore, thus  far  given  have  to  be  modified  in  practice.  Let  the 
circuit  represented  in  Fig.  104  consist  in  part  of  a  pure  re- 
sistance and  in  part  of  an  impure  inductive  circuit  or  an  imped- 


Fig.  104;— Series  circuit. 


ance,  and  let  the  voltmeters  show  the  drops  across  the  differ- 
ent parts  of  the  circuit  when  current  is  flowing.  If  then  the 
three  voltmeter  readings  be  laid  off  in  vector  diagram,  Fig.  105, 
it  will  be  found  that  the  angle  at  c  is  not  a  right  angle. 


ALTERNATING  CURRENT  AND  ITS  MEASUREMENT 


119 


The  distance  of  cd  ought  theoretically  to  represent  the  resist- 
ance drop  of  the  inductive  circuit  and  is  indeed  of  the  nature 
of  a  resistance  drop,  but  is  considerably  greater  when  obtained 


by  alternating-current  than  by  direct-current,  owing  to  hysteresis 
and  eddy  currents  in  the  inductive  apparatus. 

( h)  Current  in  A.  C.  Circuits  in  Parallel. 

When  alternating  circuits  are  arranged  parallel,  the  total  volt- 
age must  of  course  be  the  same  as  that  over  each  part. 

When  alternating-current  circuits  are  in  parallel  the  total  cur- 
rent is  the  geometric  sum  of  the  current  in  the  branches.  Fig. 


Tl         z 
1     ^0000 


Fig.  106.  — Parallel  circuit. 

106  represents  two  such  circuits,  the  one  containing  pure  re- 
sistance R,  the  other  an  impedance  Z. 

Let  the  horizontal  line  in  Fig.  107  represent  the  current  Ilt 
9 


120 


THE  MOTOR  AND  THE  DYNAMO 


shown  by  ammeter  Ax,  the  current  and  e.  m.  f.  of  this  circuit  be- 
ing in  phase.     The  phase  angle  between  !,_  and  I2  can  be  found 

Voltage  direction. 
1, 


total 


by  the  formula 


W2 


Fig.  107. 

=  cos  <f>.     Complete  the  parallelogram  and 


the  diagonal  will  be  the  value  of  Itotai  read  by  ammeter  At. 

(i)  Two-Phase  and  Three-Phase . 

It  was  seen  on  page  14  that  by  means  of  rings  tapped  on  to 
the  armature  winding  of  a  direct-current  generator  at  points 
separated  by  the  polar  span,  an  alternating  e.  m.  f.  could  be  ob- 


Ph.l. 


Fig.  108.— Two  phase. 

tained.  This  would  be  single  phase  alternating-current.  If  now 
two  more  rings  were  similarly  tapped  onto  the  armature  wind- 
ing at  points  in  quadrature  to  these,  or  at  a  phase  difference  of 
90  degrees,  the  two-phase  current  or  e.  m.  f.  lead  off  on  the  four 
wires  would  be  as  represented  in  Fig.  109. 


ALTERNATING  CURRENT  AND  ITS  MEASUREMENT 


121 


The  same  result  could  be  obtained  by  means  of  two  single 
phase  two-pole  generators  whose  shafts  are  coupled  in  a  position 
represented  by  Fig.  108.  The  relative  position  of  the  poles  in 


Z7<> 


1360 


Fig.  109.— Two  phase. 

quadrature  is  to  indicate  the  relation  in  space  of  the  two  e.  m.  f. 
curves.  The  clock  diagram  corresponding  to  Fig.  109  as  to  volt- 
age and  with  a  lagging  current  is  Fig.  1 10.  When  the  currents  in 
the  two  phases  are  equal  and  <f>  is  the  same  for  each,  the  sys- 
tem is  said  to  be  balanced.  In  such  a  case  as  this,  three  wires 
may  be  used  instead  of  four.  The  current  in  the  joint  or  middle 
wire  is  then  1.41  times  that  in  either  outside  wire,  that  being 


re*. 


pK.Z 

Fig.  no. — Two-phase  clack  diagram. 

the  ratio  of  either  side  of  the  square  to  the  diagonal.  Hence  in 
a  two-phase  alternator  the  current  capacity  of  the  armature  wind- 
ing is  41  per  cent,  more  than  if  the  same  machine  were  wound 


122 


THE;  MOTOR  AND  THE:  DYNAMO 


for  one  phase,  heating  and  energy  losses  being  the  same  in  the 
two  cases. 

If  the  direct-current  armature  and  commutator  above  referred 


Fig.  i  ii. —Three-phase  A.  Fig.  112.— Three-phase  Y. 

to  were  tapped  at  three  points  120°  apart,  a  three-phase  current 
and  voltage  could  be  obtained.  The  figures  for  three-phase  cor- 
responding to  Fig.  108  and  Fig.  109  are  Figs,  in,  112  and  113. 
In  Fig.  in  represents  what  is  known  as  the  delta  (A)  method  of 


Fig.  113. — Three-phase. 

connecting  and  Fig.  112  represents  what  is  known  as  the  star  or 
Y  method  of  connecting  the  three  distinct  armature  coils  or  wind- 
ings, as  in  Figs.  114  and  115. 
In  the  A  system  the  voltage  between  any  two  line  wires  is  that 


ALTERNATING  CURRENT  AND  ITS  MEASUREMENT 


I23 


generated  in  the  armature  coil  from  which  they  spring.     The 
current  in  each  line  wire,  however,   is  the  vector  sum  of  the 


Fig.  114.  Fig.  115. 

currents  in  the  adjacent  armature  coils.  If  the  system  is  a 
balanced  one,  the  line  current  in  each  wire  will  be  j/J  times  the 
current  in  one  armature  winding,  as  appears  from  Fig.  116. 


Fig.  116.— A-current. 

In  the  Y  system,  the  current  in  any  line  wire  is  the  same  as 
that  in  the  armature  winding  from  which  it  leads.  The  voltage, 
on  the  other  hand,  between  any  two  line  wires  is  V3  times  the 


Fig.  117.— Y-voltage. 

voltage  generated   in  one  armature   winding,   as   appears   from 
Fig.  117. 


CHAPTER  VIII. 


ALTERNATING  CURRENT  MACHINERY. 


(a)  A.  C.  Generators. 

In  polyphase  current  alternators,  except  in  very  small  machines, 
the  continuous  winding  corresponding  to  the  tapped  direct-current 
armature  above  referred  to  is  not  used.  Machines  for  the  gen- 


Fig.  118.— G.  E.  Co.  alternator  armature-(stator) winding. 

eration  of  the  commercial  alternating-current  employ  instead 
separate  windings  as  suggested  by  Figs.  108,  in  and  112.  It  is 
furthermore  usual  to  place  these  armature  windings  on  the  inner 
side  of  the  stationary  frame  of  the  machine  and  to  employ  a  ro- 


ALTERNATING   CURRENT    MACHINERY 


125 


tating  field.  The  small  exciting  direct-current  is  conveyed  to  this 
field  by  means  of  rings.  The  armature  current  then  is  drawn 
direct  from  the  windings,  without  any  brush  contacts.  See  Figs. 
118,  119,  120  and  121. 


Fig.  119. — G.  E.  Co.  alternator,  showing  core  laminations,  frame  and  field  poles. 

The  exciting  current  for  the  fields  of  large  generators  is  usually 
derived  from  a  separate  small  generator.  In  power  plants  one 
such  field  generator  may  be  used  to  supply  several  machines. 
For  isolated  alternating-current  generators  the  field  exciter  is 
sometimes  direct  coupled  to  the  shaft  of  the  alternator.  In 
smaller  machines  with  stationary  field,  a  commutator  may  be 
provided  for  field  excitation. 

(b)  Voltage  Regulation  of  the  Alternator. 

Alternators,  like  direct-current  generators,  have  the  magnetiza- 
tion curve  similarly  determined,  and  also  the  external  character- 
istic curve.  The  latter  depends  greatly  on  the  character  of  the 
load,  whether  it  be  inductive  or  non-inductive. 


126 


THE  MOTOR  AND  THE  DYNAMO 


The  voltage  regulation  of  an  alternator,  as  also  of  a  direct- 
current  generator,  is  technically  expressed  by  the  equation 

t     .  voltage  at  no  load — full  load  volts 

regulation  =  -  — — 

full  load  volts 

and  gives  the  per  cent,  rise  in  voltage  resulting  from  a  sudden 
reduction  of  the  load  to  zero. 

In  very  large  machines,  such  as  those  in  central  power  stations, 


Fig.  120.— G.  K.  Co.  three-phase  stator-winding  of  alternator. 


this  ratio  is  difficult  to  determine  by  direct  readings,  it  being 
usually  impossible  to  load  such  machines  to  their  full  capacity, 
because  of  the  difficulty  in  supplying  resistance  suitable  for 
receiving  the  full-load  current.  The  regulation  has  therefore  to 


ALTERNATING   CURRENT   MACHINERY 


127 


be  arrived  at  by  an  indirect  experimental  test.     The  problem 
will  be  made  clear  by  the  following  consideration. 

The  armature  circuit  of  the  alternator,  like  any  other  circuit, 
contains  resistance  and  inductance.    The  drop  over  each  of  these, 


Fig.  i2i.— Portion  of  stationary-armature  winding  of  a  three-phase  alternator. 

under  condition  of  any  given  load,  is  represented  in  Fig.  122  by 
IR*  and  IX«  respectively,  the  load  being  a  non-inductive  one. 

Figs  123  and  124  show  two  different  cases  with  inductive  load. 
The  first  case  is  such  that  the  impedance  of  the  armature  circuit 
increases  the  phase  angle  between  generated  volts  and  current. 
In  the  second  case  the  armature  impedance  decreases  this  angle. 


128 


THE   MOTOR  AND  THE  DYNAMO 


Ra  can  be  determined  by  the  methods  used  in  direct-current 
generators.  One  method  of  arriving  at  the  reactance  of  the 
armature  circuit  is  the  following:  With  the  field  circuit  of  the 
alternator  open,  the  armature  terminals  are  short-circuited 
through  an  ammeter  and  the  field  is  cautiously  excited  until  the 
ammeter  reads  full-load  current.  The  voltage  now  generated  is 


LOAD     VOLTS 


I  R 


Fig.  122.— Voltage  regulation  with  resistance  load. 

all  used  up  in  sending  this  current  through  the  armature  circuit. 
By  opening  the  short-circuiting  switch  and  keeping  the  speed  and 
field  excitation  constant,  the  voltage  generated  under  these  con- 
ditions may  be  read  on  a  voltmeter. 

This  voltage  includes  the  drop  at  full  load  due  to  armature 


CURRENT  DIRECTION 
Fig.  123. — Voltage  regulation  with  inductive  load. 

resistance  and  reactance  and  also  includes  the  effect  of  armature 
reaction  on  the  field  flux.  Because  the  first  of  these  three  factors 
is  very  small  and  because  the  third  has  an  effect  on  the  power 
factor  of  the  machine  similar  to  reactance,  the  voltage  thus 


ALTERNATING   CURRENT    MACHINERY 


129 


obtained  is  called  the  drop  due  to  synchronous  reactance  and 
may  be  considered  the  IXrt  of' the  preceding^diagrams. 

The  method  here  given,  known  as  the  electro-motive-force 
method  of  determining  voltage  regulation,  is  not  an  accurate  one, 
owing  to  the  fact  that  the  field  being  necessarily  weak,  the  arma- 
ture reaction  is  excessive.  In  a  polyphase  machine,  however,  the 
result  is  likely  to  be  nearer  the  true  value  than  in  a  single-phase 


J-B 


CURRENT  DIRECTION 


Fig.  124. 


generator.  For  a  more  elaborate  treatise  on  this  subject  than 
is  allowed  by  the  scope  of  this  book  the  reader  is  referred  to 
Thomalen's  "Electrical  Engineering." 

(c)  The  Inductor  Alternator. 

The  inductor  alternator  differs  from  all  other  types  of  electric 
generator  in  that  the  windings,  both  field  and  armature,  are 
stationary,  and  the  iron  alone  revolves.  The  armature  is  wound 
on  the  frame,  as  in  other  alternators,  and  the  field  cores  consti- 


130 


THE  MOTOR  AND  THE)  DYNAMO 


tute  the  rotating  part.  This  machine,  therefore,  is  very  rugged 
in  construction.  Its  alternating-current  wave  is  almost  a  pure 
sine  curve,  and  in  power-factor  and  efficiency  it  compares  favor- 
ably with  the  ordinary  type  of  alternating-current  generator.  It 


Fig.  125. — Inductor  alternator  with  vertically  split  armature. 

is  especially  well  adapted  for  a  widely  varying  load  at  low  voltage, 
as  is  demanded  for  electric  welding,  etc.     See  Fig.  125. 

(d)  The  Compounding  of  Alternators. 

A  series  winding  is  sometimes  added  to  the  field  of  an  alterna- 
tor for  the  purpose  of  maintaining  a  constant  or  an  increasing 
voltage  with  increase  of  load.  Formerly,  one  method  largely  em- 
ployed consisted  in  shunting  off  a  portion  of  the  main  armature 
current  and  passing  it  through  a  rectifier.  This  was  simply  a 
commutator  having  as  many  segments  as  there  were  field  poles 
and  mounted  on  the  shaft  of  the  machine.  The  brushes  took  off 


ALTERNATING   CURRENT    MACHINERY  13! 

a  pulsating  current  which  supplied  the  series  field  and  varied  with 
the  main  armature  current. 

The  modern  method  is  to  vary  automatically  the  voltage  im- 
pressed on  the  field  of  the  alternator  by  its  direct-current  exciter. 
The  automatic  device  controlling  this  operation  is  termed  the 
Tirrell  regulator,  and  the  connections  are  shown  in  Fig.  126. 
Briefly 'its  operation  is  as  follows: 

When  the  exciter  voltage  falls  too  low,  the  direct-current  con- 
trol-magnet on  the  left  is  weakened.  When  the  alternating-cur- 
rent generator  voltage  falls,  the  solenoid  magnet  to  the  right  is 


Afa/n Contacts 


AC  fie/d        >4  C  Generator 
Rheostat 

ELEMENTARY  DIAGRAM  OF  TA.  FORM  A  REGULATOR 
Fig.  126.    G.  E.  Co. 

weakened.  Either  or  both  of  these  operations  close  the  main  con- 
tacts, according  to  the  adjustments  of  the  counterweight.  This  neu- 
tralizes the  relay  magnet,  closing  the  relay  contacts,  and  so  short- 
circuits  the  exciter  field  rheostat  and  raises  the  exciter  voltage.  This 
increase  of  voltage  re-opens  the  main  contacts.  "The  operation  is 
continued  at  a  high  rate  of  vibration,  due  to  the  sensitiveness  of 
the  control-magnets,  and  maintains  not  constant  but  a  steady 
exciter  voltage."  A  compensating  winding  on  the  alternating- 
current  control  magnet  is  connected  to  a  current  transformer  in 
the  main  line  and  causes  an  increase  of  voltage  with  increase  of 
load,  thus  taking  care  of  the  line-drop. 


132 


THE  MOTOR  AND  THE  DYNAMO 


(e)  The  Synchronous  Motor. 

In  treating  of  the  direct-current  motor,  it  was  shown  that  rota- 
tion is  produced  by  the  action  of  the  field  flux  on  the  current- 
bearing  armature  conductors.  In  order  that  this  thrust  may  pro- 
duce continuous  rotation,  a  commutator  is  required  to  change  the 
direction  of  the  current  in  each  conductor  twice  in  each  cycle, 
that  is,  to  produce  in  the  conductor  an  alternating  current  whose 
direction  changes  simultaneously  with  the  passage  of  the  con- 
ductor from  pole  to  pole.  If  therefore  the  field  continues  to  be 


Fig.  127. — A  large  alternator  in  process  of  construction. 

excited  with  a  direct  current,  and  the  commutator  of  such  a  motor 
be  replaced  by  rings,  and  an  alternating  current  of  the  proper  fre- 
quency be  fed  into  the  rotating  armature,  the  motor  will  continue 
to  run  at  a  constant  speed  in  synchronism  with  the  alternating 
current.  To  operate  such  a  motor  on  the  alternating  current,  it 
must  obviously  first  be  brought  up  to  speed  and  also  into  the 


ALTERNATING  CURRENT    MACHINERY 


133 


correct  phase  relation  with  the  given  current.  Should  it  be 
slowed  down  so  as  to  fall  out  of  step  with  the  given  alternating 
current  source  of  supply,  it  will  immediately  stop. 

Fig.  128  shows  a  device  used  in  starting  such  motars.    It  con- 


SYNCH 


Fig.  128.— Connections  for  synchronous  motor. 

sists  of  lamps  bridged  across  the  switch  between  the  motor  and 
its  generator.  These  lamps  prevent  the  flow  of  an  excessive 
current  and  serve  to  indicate  the  relative  frequency  and  the  phase 

(a)  (b) 


relation  of  the  two  machines.  Let  the  generator  be  driven  by 
its  engine  or  other  prime  mover  at  a  definite  and  constant  speed, 
and  let  the  motor  be  started  by  some  device,  say  a  small  direct- 
current  motor,  whose  speed  can  be  regulated.  Each  machine  will 
now  be  generating  an  alternating-current  e.  m.  f.  If  these 
e.  m.  f.'s  are  opposed  to  each  other  in  direction  through  the  lamps, 
it  is  evident  that  no  current  will  flow  between  the  machines,  and 


134  THE  MOTOR  AND  THE  DYNAMO 

the  lamps  will  be  dark.  The  e.  m.  f /s  are  opposed  to  each  other 
in  phase  (see  (a)  ).  If  on  the  other  hand,  the  e.  m.  f.'s  are  so 
related  as  to  send  the  current  in  the  same  direction  through  the 
lamps,  they  correspond  in  phase  as  regards  this  circuit  (see  (b)  ), 
and  the  result  will  be  an  increased  voltage  across  the  lamps  caus- 
ing them  to  glow.  If  either  machine  has  a  different  frequency 
from  the  other,  there  will  result  alternate  reinforcement  and 
interference,  and  the  lamps  will  flicker  as  in  (c), 

The  method  is  to  regulate  the  speed  of  the  motor  so  that  the 
flickering  becomes  very  slow,  and  then  to  close  the  switch  in  the 
middle  of  a  dark  period.  This  may  be  varied  by  having  the 
lamps  cross-connected.  The  switch  should  then  be  closed  in  the 


I' 


o 

Fig.  129.— Vectordiagram  of  synchronous  motor. 

middle  of  a  bright  period.  If  now  the  starting  device  be  mechan- 
ically disconnected  or  its  driving  circuit  opened,  the  synchronous 
motor  will  continue  to  operate  on  the  alternating-current  fed  into 
it.  This  will  be  clearer  if  the  e.  m.  f .  generated  by  the  motor  in 
starting  be  considered  its  natural  counter  e.  m.  f .,  as  in  the  direct- 
current  motor,  which  is  opposed  in  direction  to  the  impressed 
e.  m.  f.  of  the  driving  circuit. 

It  will  be  remembered  that  the  direct-current  motor  draws  cur- 
rent in  proportion  to  the  load  put  upon  it,  because  of  the  retarda- 
tion in  speed  caused  by  the  load.  In  the  synchronous  motor, 
being  a  constant  speed  machine,  this  cannot  be  the  case.  The 
operation  of  this  motor  is  to  be  explained  by  the  vector  diagram, 
Fig.  129. 


ALTERNATING    CURRENT    MACHINERY  135 

L,et  the  e.  m.  f.  of  the  generator  E.  be  considered  positive  and 
so  plotted  toward  the  right  from  the  origin  O.  The  counter 
e.  m.  f  the  motor  E,*  will  then  extend  toward  the  left.  But 
although  at  the  instant  of  connection  to  the  circuit  the  motor  may 
be  in  direct  opposition  to  the  generator,  when  its  starting  device 
is  cut  off,  it  instantly  falls  somewhat  behind  the  180°  phase;  that 
is,  its  vector  will  take  the  direction  OE,«.  These  e.  m.  f.'s  are 
in  a  series  circuit.  Following  the  usual  method  of  combining 
e.  m.  f.'s  in  series  by  completing  the  parallelogram,  we  have  the 
resultant  e.  m.  f.  OE^,  which  sends  the  driving  current  through 
the  motor  armature.  Because  of  the  impedance  of  the  motor 
circuit,  this  is  a  lagging  current,  or  OI,  the  cos^>  depending  solely 
on  the  character  of  the  motor  circuit  and  being  therefore  a 
constant. 

Now  suppose  the  load  on  the  motor  to  be  increased.     This 


I 
'I" 

Fig.  130. — Vector  diagram  of  synchronous  motor. 

tends  to  cause  a  slackening  of  .speed,  but  what  really  happens  is 
that  E,«  swings  into  a  new  relation  with  EAO  namely  Ew',  giving 
a  new  resultant  E/,  and  current  I'.  The  impedance  of  the  cir- 
cuit being  essentially  constant,  I  increases  with  the  increase  in  Er. 
It  will  be  remembered  that  the  speed  of  a  D.  C.  shunt  motor 
may  be  increased  by  resistance  in  the  field  circuit.  A  change  in 
the  field  current  of  the  synchronous  motor  serves  only  to  shift  the 
phase  relation  between  Ee  and  E,M.  In  Fig.  1 30  the  vectors  OE^  and 
OI  correspond  in  direction,  and  the  power  factor  of  the  driving 
current  is  unity.  Should  E,«  be  increased,  by  an  increase  in  the 
motor  field  current,  to  E,,/,  then  Er  and  I  would  have  new  posi- 
10 


136  THE  MOTOR  AND  THE  DYNAMO 

tions,  namely  E/,  and  I'.  Likewise  should  E,«  be  decreased  to 
E,,/',  then  Er  and  I  would  become  E/'  and  I".  The  lower  values 
of  K,«  therefore  cause  the  current  from  the  generator  to  be  a  lag- 
ging current  and  the  higher  values  of  E,«  cause  the  synchronous 
motor  to  have  the  effect  of  a  capacity,  giving  the  generated  cur- 
tent  a  position  of  lead. 

For  any  given  power  input  into  the  motor,  there  is  one  value 
of  motor  field  at  which  the  driving  current  is  a  minimum,  a 
change  either  way  causing  an  increase  in  the  current  for  the  same 
amount  of  power  delivered  to  the  motor.  This  gives  rise  to  the 
so-called  V  curves  plotted  between  motor  current  and  motor 
e.  m.  f . 

A  synchronous  motor  with  over-excited  field  may  be  used  to 
improve  the  power  factor  of  a  transmission  line. 

An  inspection  of  Fig.  129  will  make  it  evident  that  the  syn- 
chronous motor  is  limited  in  the  load  it  can  carry  without  pulling 
out  of  step.  Ew  can  shift  phase  with  respect  to  E^  until  the 
angle  between  E,«  and  motor  current  I  is  90°.  This  means  a 
current  of  zero  power  factor,  and  the  motor  will  stop  when  this 
angular  relation  is  reached.  The  stopping  of  the  synchronous 
motor  is  sudden,  when  it  has  pulled  out  of  phase.  The  actual 
operative  range  of  the  synchronous  motor  is  over  a  smaller  angle 
than  that  indicated  by  this  90°  limit,  because  of  internal  losses 
in  the  machine.  The  stopping  of  the  motor  under  these  condi- 
tions is  the  same  as  would  happen  in  a  direct-current  machine, 
if  commutation  were  to  occur  not  on  an  axis  at  right  angles  to 
the  field  flux  but  90°  removed  from  this  point. 

(f )  The  Operation  of  A.  C.  Generators  in  Parallel. 

Instead  of  generator  and  synchronous  motor,  consider  the  two 
machines  just  under  discussion  to  be  two  alternating-current  gen- 
erators. Let  one  generator  be  furnishing  current  to  a  circuit. 
The  other  generator  can  be  brought  up  to  speed  and  voltage, 
synchronized  and  connected  to  the  bus  bars  in  the  same  manner 
as  the  synchronous  motor.  When  once  so  running  and  connected, 
two  alternators  tend  to  remain  in  step.  For  let  one  of  them  be 
supposed  to  drop  behind,  it  immediately  becomes  a  synchronous 


ALTERNATING   CURRENT    MACHINERY  137 

motor,  the  resultant  e.  m.  f.,  E,-  of  Fig.  129,  sending  a  current 
through  its  armature.  This  relieves  the  load  on  its  driving  engine 
and  tends  at  the  same  time  to  retard  the  other  machine,  so  that 
the  two  swing  again  into  step.  This  very  action,  however,  has 
proved  to  be  a  source  of  great  trouble  when  the  governors  of 
the  engines  are  of  quick  action.  For  in  that  case  it  is  found 
that  the  heavily  loaded  machine  is  immediately  supplied  with  more 
power,  the  driven  one  with  less,  which  interferes  with  their 
natural  tendency  to  fall  again  into  step.  Heavy  fly-wheels  also 
interfere  with  this  tendency  to  remain  in  synchronism  by  carry- 
ing the  retarded  or  aided  machine  beyond  its  proper  position  and 
so  leading  to  the  phenomenon  known  as  "hunting." 

It  will  be  recalled  that  when  direct-current  generators  are 
operated  in  parallel,  the  distribution  of  the  load  between  the  two 
machines  is  regulated  by  their  field  rheostats,  controlling  their 
relative  e.  m.  f 's.  This  is  not  the  case  with  alternators.  An 
increase  of  voltage  of  either  generator  gives  rise  to  a  shift  in 
the  phase  relation  of  the  two  machines  and  a  useless  cross-current 
between  them.  There  is  therefore  one  position  of  the  field  rheo- 
stat for  each  alternator  operating  in  parallel  with  others  such 
that  the  sum  of  all  the  currents  shall  just  equal  the  total  output 
of  the  station.  A  change  in  either  direction  from  this  position 
indicates  an  uneconomical  adjustment  and  a  loss  of  power. 

The  means  of  controlling  the  output  of  the  various  alternators 
in  parallel  connection  is  to  be  sought  therefore  in  the  power  sup- 
plied to  the  prime  movers,  as,  for  instance,  the  steam  supplied 
to  the  engines.  By  regulating  this,  the  load  on  the  various  gen- 
erators can  be  controlled. 

For  synchronizing  generators  in  power  stations,  preparatory 
to  throwing  a  machine  into  service,  a  device  with  a  hand  and 
dial  known  as  a  synchroscope  is  generally  employed  in  place  of 
the  less  reliable  lamps.  The  direction  of  rotation  of  the  hand 
indicates  whether  the  machine  to  be  connected  is  running  too 
fast  or  too  slow.  A  stationary  hand  indicates  perfect  synchronism 
and  a  hand  stationary  in  a  vertical  position  indicates  perfect 
synchronism  and  equality  of  phase.  See  Fig.  131. 


138  THE  MOTOR  AND  THE  DYNAMO 

As  regards  the  motive  power  of  alternating-current  generators 


Fig.  131.— Synchronism  indicator.    G.  E.  Co. 

operating  in  parallel,  reciprocating  steam  engines  are  still  found 
in  many  large  plants.  But  because  of  the  irregularities  in  the 
period  of  rotation  of  any  reciprocating  engine  due  to  the  very 


Fig.  132.— Crocker-Wheeler  Co.  a.c.  generators  driven  by  reciprocating  steam  engines. 

nature  of  its  construction,  turbines,  both  steam  and  water,  seem 


ALTERNATING   CURRENT    MACHINERY 


139 


Fig.  133. — G.  E.  Co.  8,000  k.w.  a.c.  generators  with  vertical  shaft  operated  by 
Curtis  steam  turbines. 


Fig.  134.— Crocker-Wheeler  Co.  4,000  k.v.a.  generators  operated  by  gas  engines. 


140 


THE;  MOTOR  AND  THE  DYNAMO 


now  to  be  preferred  as  prime-movers  for  alternators.  Especially 
water-power  drive  has  been  rendered  very  perfect  to-day  by  the 
invention  of  an  oil-pressure  governor.  The  oil  is  kept  under 
pressure  by  a  pump,  either  controlled  or  directly  operated  by  the 
turbines.  A  change  in  speed  of  any  generator  and  turbine  results 
in  a  change  of  oil-pressure.  By  a  system  of  pistons  and  corn- 


Fig.  135.— 4,000  k.w.,  2,2oo-volt  generators  at  Niagara  Falls.    G.  E-  Co. 

pound  levers,  this  change  of  pressure  is  made  to  operate  the 
guide-vanes  of  the  water-wheels,  thus  controlling  the  direction 
and  amount  of  the  water  entering  the  wheel.  This  system  is  in 
use  in  the  new  power-house  on  the  River  Rhine  near  Basel  and  is 
being  installed  in  the  great  power-house  now  under  construction 
at  Keokuk  on  the  Mississippi  River. 

(g)  The  Rotary  Converter. 

The  so-called  rotary  converter  is  essentially  a  shunt  generator 
or  motor  with  the  usual  commutator  mounted  at  one  end  of  the 


ALTERNATING    CURRENT    MACHINERY 


141 


armature  and  with  the  rings  tapped  on  according  to  the  number 
of  poles  and  of  phases.  For  instance,  in  the  central  distance 
between  two  like  field  poles  there  must  be  two  taps,  one  to  each 
ring  for  single  phase,  three  for  three  phase  and  four  for  two 
phase,  in  each  case  equally  spaced.  See  Fig.  136. 


Fig.  136. — Rotary-converter  armature  connection. 

The  machine  may  then  be  run  from  the  direct-current  end  and 
be  made  to  furnish  alternating-current  or  from  the  alternating- 
current  end  and  furnish  direct-current.  See  Figs.  137,  138  and 
139.  The  latter  method  is  the  usual  one.  When  run  from  the 
direct-current  end,  the  machine  is  spoken  of  as  an  "inverted 
rotary." 


142 


THE:  MOTOR  AND  THE;  DYNAMO 


The  chief  use  of  the  rotary  converter  is  to  be  found  in  the 
sub-stations  of  light  and  power  companies.  In  order  to  minimize 
expense  of  transmission,  it  is  the  custom  to  generate  power  in  the 
central  station  at  a  high  alternating-current  e.  m.  f .  This  can 
be  transported  to  great  distances  over  comparatively  small  wires. 
In  order  to  be  of  commercial  value,  it  must  be  stepped  down  to  a 
lower  voltage,  which  is  easily  accomplished  by  means  of  the 


Fig.  137.— G.  E.  Co.— A  modern  rotary  converter.    D.  C.  end. 

transformer.  This  instrument  will  receive  a  brief  notice  later. 
But  the  transformer  delivers  the  power  in  the  alternating-current 
form.  This  is  serviceable  for  both  incandescent  and  arc  lighting 
and  for  operating  certain  types  of  motors.  There  are,  on  the 
other  hand,  applications  to  which  direct-current  is  much  better 
fitted  than  alternating-current,  as,  for  instance,  traction  machinery 
and  motors  of  varying  and  controllable  speed.  Hence  in  the  sub- 
stations of  traction  companies  the  high  pressure  alternating  cur- 
rent received  from  the  main  power-house  is  usually  stepped  down 


ALTERNATING   CURRENT    MACHINERY 


143 


by  means  of  a  transformer  and  then  supplied  to  the  alternating- 
current  end  of  a  rotary  converter,  from  which  direct-current  is 
sent  out  over  the  feeders  to  different  points  of  the  system. 

The  behavior  of  the  rotary  converter,  when  operated  from  the 
alternating-current  end,  is  very  similar  to  that  of  the  synchronous 
motor.  If  directly  connected  to  the  line,  without  the  interven- 
tion of  a  transformer  or  other  inductive  circuit,  a  variaton  of 


Fig.  138.— G.  E.  Co.  rotary  converter.    A.  C.  ends. 

field  excitation  will  produce  a  shift  of  phase  with  reference  to 
the  supply  voltage,  but  will  not  much  alter  the  direct-current 
volts  delivered.  As  in  the  synchronous  motor,  the  amount  of 
field  excitation  also  determines  the  phase  relation  between  the 
current  and  the  impressed  alternating-current  e.  m.  f.  In  case 
there  is  inductance  in  the  supply  circuit,  as,  for  instance,  a  trans- 


144 


THE  MOTOR  AND  THE  DYNAMO 


former,  a  change  of  field  current  may  also  vary  the  generated 
counter  e.  m.  f.  and  therefore  the  direct-current  volts,  but  not 
over  a  wide  range.  In  the  machine  shown  in  Fig.  139,  the  most 


Fig.  139.— G.  IJ.  Co.  regulating-pole  rotary  converter. 

recent  type,  the  direct  current  volts  are  successfully  regulated 
over  a  range  of  10  per  cent,  in  either  direction  by  means  of  sepa- 
rate regulating  poles.  When  operated  from  the  direct-current 


ALTERNATING   CURRENT    MACHINERY  145 

end,  a  change  of  field  current,  although  it  may  increase  or 
decrease  the  speed  as  in  a  shunt  motor,  has  no  effect  on  the 
alternating-current  volts  unless  the  machine  is  loaded,  and  then 
only  a  slight  one. 

The  theoretical  relationship  between  direct-current  and  alter- 
nating-current volts,  whether  the  machine  is  operated  from  either 
end  or  is  driven  as  a  generator  by  some  prime  mover,  would  be 
as  follows: — 

D.  C.  A.  C. 

Single  phase 100  70.7 

Three  phase 100  61.2 

Two  phase 100  50.0 

This  table  is  on  the  basis  of  a  sinusoidal  e.  m.  f.  In  the  actual 
converter,  however,  this  condition  is  never  realized,  and  the  ratio 
between  direct-current  and  alternating-current  e.  m.  f.'s  varies 
considerably  from  these  values,  even  at  zero  load. 

When  loaded,  the  armature  of  the  rotary  carries  both  alter- 
nating-current and  direct-current.  In  any  armature  conductor 
the  external  direct-current  becomes  alternating  through  the 
agency  of  the  commutator.  Thus  each  conductor  has  in  it  two 
alternating-current  components  of  current,  the  one  theoret- 
ically sinusoidal,  the  other  with  more  abrupt  rise  and  reversal. 
And  these  two  components  are  not  in  phase.  The  wave-shape  of 
the  resulting  current  therefore  departs  altogether  from  the  sinu- 
soidal form. 

From  the  foregoing  paragraphs  it  will  be  clear  that  there  must 
be  two  forms  of  armature  reaction  present  in  the  loaded  machine. 
The  one  distorts  the  field  flux  in  a  forward  direction,  the  other 
against  the  direction  of  rotation.  The  direct-current  brushes  are 
therefore  to  be  set  on  the  geometrical  axis  of  symmetry  between 
the  poles. 

When  the  machine  is  operated  from  the  direct-current  end  and 
an  inductive  load  is  placed  on  the  alternating-current  end,  the 
lagging  causes  a  field  weakening  and  an  increase  in  speed  which 
may  attain  dangerous  proportions. 

In  sub-stations  where  there  are  several  rotary  converters,  it  is 
the  custom  to  start  the  machines  from  the  direct-current  end, 


146 


THE  MOTOR  AND  THE  DYNAMO 


when  they  can  readily  be  brought  up  to  speed  and  synchronized. 
As  one  or  more  machines  are  always  in  operation,  the  requisite 
direct-current  supply  is  always  at  hand. 

Rotary  converters  have  the  fault  of  hunting,  just  as  alternators 
when  operated  in  parallel.  In  both  machines  this  is  to  a  great 
extent  obviated  by  inserting  heavy  copper  conductors  of  various 

forms,   I  \     J,  etc.,  in  the  faces  of  the  pole  pieces,  the 

pole  pieces  themselves  being  laminated.    A  heavy  steel  shoe  across 


Fig.  140.— I^arge  rotary  converter  in  process  of  construction.    Transformers 
in  the  distance. 

the  face  of  the  laminated  pole  piece  has  a  similar  effect.  The 
eddy-currents  set  up  in  these  give  steadiness  of  motion  and  pre- 
vent hunting. 

(h)  The  Transformer. 

Although  the  transformer  is  neither  motor  nor  dynamo,  yet  a 
brief  notice  of  this  device  must  be  included  in  the  present  volume. 


ALTERNATING   CURRENT    MACHINERY  147 

It  consists  of  two  spools,  or  windings,  of  insulated  wire  placed 
on  a  core  of  laminated  iron  or  surrounded  by  the  same,  and 
usually  immersed  in  oil  contained  in  an  iron  casing.  The  func- 
tion of  the  oil  is  to  cool  and  insulate.  Forms  are  shown  in  Figs. 
141,  142  and  143.  The  alternating  flux  created  by  the  one  coil 
cuts  the  other  coil,  establishing  in  it  an  alternating  e.  m.  f . 

The  coils  are  known  as  primary  and  secondary,  and  ignoring 


Fig.  141.— The  transformer.    G.  E.  Co. 

losses  the  respective  voltages  are  in  direct  ratio  to  the  number  of 
turns  in  the  coils,  the  currents  being  in  the  inverse  ratio.  The  high 
voltage  coil  is  usually  termed  the  primary,  being  the  one  to  which 
the  power  is  furnished,  the  transformer  being  chiefly  used  to 
step  down  the  voltage  of  a  transmission  line.  In  power  stations, 
however,  the  instrument  is  sometimes  used  to  step  up  the  voltage. 
It  is  the  transformer  which  furnishes  excuse  for  the  existence 
of  the  alternating  current  as  a  commercial  form  of  power.  Power 


148 


THF,  MOTOR  AND  THE  DYNAMO 


at  a  high  voltage  with  small  current  is  far  less  expensive  as 
regards  copper  and  PR  loss  when  transmitted  to  considerable 
distances  than  the  same  power  at  low  voltage  with  large  current. 


Fig.  142.— Wagner  lo-kw.  lighting  transformer  element. 

The  transformers  more  than  pay  for  themselves  on  such  a  high 
voltage  line,  and  their  efficiency  of  operation  is  high,  from  some 
94  to  98  per  cent.    The  ratio  commonly  used  is  ten  to  one. 
The  losses  in  transformers  are  therefore  small.    The  iron  loss, 


ALTERNATING    CURRENT    MACHINERY  149 

that  is,  the  power  used  for  overcoming  hysteresis  and  eddy- 
currents  in  the  core  or  shell,  as  the  case  may  be,  ranges  from 
0.6  per  cent,  to  i  per  cent,  in  modern  transformers  and  are  prac- 
tically constant  for  all  loads.  The  copper  losses,  PR,  of  both 
windings,  range  from  i.i  to  1.8  per  cent. 

The  no  load,  or  exciting,  current  is  very  small,  owing  to  the 


Fig.  143. — Automatic  constant  current  transformer  for  series 
lighting  system.     G.  E.  Co. 

c.  e.  m.  f.  of  self-induction  in  the  primary  circuit.  The  power 
factor  at  no  load  is  also  very  small,  the  resistance  being  compara- 
tively low  and  the  reactance  high.  The  exciting  current,  there- 
fore, lags  almost  9x3°  behind  the  primary  volts.  When  the  load 
current  is  drawn  from  the  secondary  windings,  however,  it  is  at 
the  expense  of  the  primary  flux,  tending  to  reduce  it.  This  in 


THE;  MOTOR  AND  THE;  DYNAMO 


turn  tends  to  lower  the  c.  e.  m.  f .  of  self-induction  and  allows 
the  primary  current  to  increase  in  value  in  proportion  to  the  load. 
The  flux  of  the  core,  or  shell,  is  in  this  way  restored  and  remains 
constant  in  value  throughout  all  loads,  and  the  transformer  draws 
power  automatically  from  the  mains,  according  to  the  demands 
made  upon  it. 

Since  the  capacity  of  the  transformer  depends  on  temperature, 
and  the  heat  developed  in  the  coils  depends  on  current  independent 
of  power-factor,  transformer  capacity  is  usually  expressed  in 
kilo-volt  amperes,  K.  V.  A.,  rather  than  in  kilowatts.  In  sizes 


100  volts 


12.5   volts 

Fig.  144. — The  auto  transformer. 

over  40  K.  V.  A.  the  retaining  case  is  usually  corrugated  to  aid 
the  cooling,  and  in  the  largest  sizes  recourse  is  had  to  the  air- 
blast  or  to  water  circulation. 

The  auto  transformer  consists  of  a  single  turn  on  a  laminated 
core,  and  its  action  is  much  like  that  of  the  potentiometer  in 
direct-current  service.  Unlike  this  instrument,  however,  voltage 
may  be  stepped  up  as  well  as  reduced  by  the  auto  transformer. 
See  Fig.  144. 

(i)  The  Induction  Motor. 

The  induction  motor  receives  its  name  from  the  fact  that  the 
current  in  what  corresponds  to  the  armature  is  not  drawn  from 
the  supply  mains  but  is  induced.  The  armature,  more  properly 
called  the  rotor,  is  not  electrically  connected  with  the  outside 
source  of  supply.  Its  current  is  generated  by  the  alternating  flux 
from  the  poles  of  the  stationary  part,  or  stator. 

In  Fig.  145  the  stator  is  wound  for  a  two-phase  four-wire  cir- 
cuit, having  two  poles  to  each  phase.  There  are  thus  two  dis- 
tinct windings.  Winding  AC  creates  a  flux  such  that  the  stator 


ALTERNATING   CURRENT    MACHINERY  151 

iron  has  a  north  pole  at  b  and  a  south  pole  at  d.  The  winding 
BD  on  the  other  phase  is  at  this  time  dead.  As  the  current  in 
AC  dies  out,  that  in  winding  BD  builds  up  (see  Fig.  146),  so 
that  the  north  pole  is  gradually  shifted  from  b  to  c  and  the  south 
pole  from  d  to  a.  In  the  second  quarter  of  the  cycle,  the  current 
in  winding  CA  builds  up  in  the  reverse  direction,  that  in  BD 


Ph.a 


Fig.  145.— The  induction  motor.    Diagram. 

dying  out,  thus  shifting  the  north  pole  to  the  positions  d  and  the 
south  pole  to  b.  By  continuing  this  analysis  through  the  next 
two  quarters,  it  will  be  seen  that  the  polarity  of  the  stator  iron  is 
made  to  rotate,  in  this  instance  once  around  for  a  complete  cycle. 
Were  there  four  poles  per  phase,  it  is  evident  that  it  would 
require  two  complete  cycles  to  cause  the  polarity  of  the  stator 
ii 


152  THE  MOTOR  AND  THE  DYNAMO 

iron   to  make  a  complete  revolution.     That  is,   the  number  of 

revolutions  of  the  stator  magnetism  per  secoud,  n,  is  — —, —  where 

Yzp 

f  is  the  frequency  and  />  the  number  of  poles  per  phase,  or 

•-f- 

A  rotor  consisting  of  a  solid  cylinder  of  iron  would  experience 
the  drag  of  this  flux,  and  if  the  friction  were  not  too  great,  would 
rotate  in  synchronism  with  it.  A  cylinder  or  other  centrally 
symmetrical  form  of  any  metal  would  also  be  put  in  motion 
because  of  the  eddy-currents  set  up  in  it  by  the  stator  flux. 

The  rotor  of  the  induction  motor  consists  of  the  usual  laminated 
iron  core,  either  wound  similarly  to  the  armature  of  an  alternator 
or  pierced  near  its  periphery  by  stout  copper  bars  connected  in 


B 


C' 

Fig.  146. 

The  latter  is  known  as  a  squirrel-cage  rotor  in  distinction 
from  the  wound  rotor.  By  a  reference  to  Fig.  145  it  will  be 
seen  that  these  windings  or  bars  are  not  unlike  the  secondary 
windings  of  a  transformer,  and  have  for  the  same  reason  an 
alternating  e.  m.  f .  induced  in  them.  It  is  the  resulting  current 
on  which  the  stator  flux  acts  in  producing  rotation.  And  because 
the  windings  or  bars  give  the  proper  direction  to  this  induced 
current,  the  torque  effected  is  much  greater  than  could  possibly 
be  the  torque  on  a  solid  metal  cylinder. 

If  the  speed  of  the  rotor  be  such  that  it  is  in  exact  synchronism 
with  the  rotating  stator  flux,  then  the  rotor  conductors  will  be 
fixed  in  their  relation  to  this  flux,  and  no  current  will  be  induced 
in  them.  As  a  rule,  however,  the  speed  of  the  rotor  is  slightly 
less  than  that  of  the  stator  flux,  the  ratio  of  this  difference  to 


ALTERNATING   CURRENT    MACHINERY  153 

the  speed  of  the  stator  flux  being  technically  known  as  the  slip; 
that  is, 

r.p.m.  of  stator — r.p.m.  of  rotor 
%  slip  =  -  — —  -  X  ioo. 

r.p.m.  of  stator 

The  greater  the  load  on  the  pulley  wheel,  the  greater  the  slip, 
and  the  more  rapid  is  the  cutting  of  the  rotor  conductors  through 
the  stator  flux,  and  the  higher  the  e.  m.  f .  and  consequent  current 
induced  in  them. 

The  increase  of  current  in  the  rotor  which  accompanies  an 
increase  of  load  supply  calls  for  additional  power  supply.  This 
is  furnished  to  the  stator  by  the  mains  automatically,  the  process 
being  similar  to  that  by  which  a  load  on  the  secondary  of  a 
transformer  (the  rotor)  increases  the  current  supply  to  the  pri- 
mary (the  stator).  When  the  rotor  is  at  rest,  the  induction  motor 
is  essentially  a  transformer  with  an  unusually  large  factor  of 
magnetic  leakage.  It  is  the  factor  of  magnetic  leakage  coupled 
with  rotor  resistance  and  inductance  which  prevents  the  rotor 
current  from  becoming  excessive  at  stand-still.  This  effect  in 
turn  prevents  the  stator  current  from  rising  to  a  dangerous  value 
at  stand-still,  as  it  would  in  a  transformer  with  short-circuited 
secondary. 

The  question  of  rotor  speed  and  the  torque  exerted  by  an 
induction  motor  is  a  very  complicated  one,  and  the  various  gov- 
erning factors  must  be  taken  up  in  detail.  First  suppose  the 
torque  demanded  at  the  pulley  wheel  to  be  doubled.  When  this 
has  slowed  down  the  speed  to  such  a  degree  that  the  slip  has 
been  doubled,  the  rotor  current,  considering  resistance  alone,  is 
also  doubled,  furnishing  the  required  torque. 

But  other  factors  enter  to  disturb  this  simple  ratio  between 
slip  and  torque.  By  the  increased  slip,  the  frequency  of  the 
rotor  current  and  consequently  the  rotor  reactance  (2w/L)  is 
increased.  This  both  reduces  the  current  somewhat  and  changes 
its  position  with  reference  to  the  field  flux,  causing  a  greater 
rotor  (armature)  reaction  than  otherwise  would  be  the  case  and 
augmenting  the  flux  leakage.  All  these  factors  combine  to  make 
the  slip  considerably  more  than  twice  as  great  for  double  torque. 


MOTOR  AND  THE  DYNAMO 


The  rotor  reaction  in  particular  is  considerable  and  so  distorts 
the  stator  flux  that  a  large  part  of  it  passes  from  pole  to  pole 
through  the  narrow  clearance  space  between  rotor  and  stator  in 
such  a  way  as  not  to  act  on  the  rotor  inductors. 

Thus  after  a  certain  point  is  reached  the  speed  of  the  induction 
motor  falls  off  rapidly  with  increase  of  load ;  and  at  a  point  known 
as  the  pull-out  torque,  the  motor  stops  altogether. 

It  will  be  seen,  therefore,  that  the  induction  motor  is  similar 
to  the  direct-current  motor  in  the  fact  that  a  decrease  of  speed 
due  to  load  immediately  causes  an  increase  of  current  drawn  from 
the  mains.  It  is  unlike  a  direct-current  motor,  on  the  other 


Per  cent.  slip. 

Fig.  147.— Curve  of  induction  motor. 

hand,  in  that  when  a  certain  point  is  reached,  the  speed  falls  off 
rapidly,  and  a  torque  of  some  100  per  cent,  more  than  that  for 
which  the  motor  is  rated  will  in  most  cases  bring  the  machine 
to  a  stand-still. 

For  any  given  value  of  the  slip,  the  torque  of  an  induction  motor 
varies  as  the  square  of  the  voltage.  This  comes  about  from  the 
fact  that  the  torque  is  proportional  to  the  product  of  stator  flux 
and  rotor  current.  An  increase  of  applied  voltage  increases  this 
flux  and  this  in  turn  increases  the  rotor  current  an  equal  amount, 
hence  the  square.  The  converse  of  this  proposition  is  that  with 


ALTERNATING   CURRENT   MACHINERY 


155 


torque  constant,  the  slip  will  vary  inversely  with  the  square  of 
the  applied  voltage. 

The  general  shape  of  the  speed  torque  curve  of  an  induction 
motor  may  be  seen  in  Fig.  147.  It  will  be  observed  that  at  about 
35  per  cent,  slip  in  this  case  the  torque  suddenly  begins  to  fall 
off,  meaning  that  the  so-called  pull-out  torque  has  been  reached, 
and  the  motor  is  stopping.  A  means  of  shifting  this  turning 
point  to  correspond  to  a  slower  speed  is  to  put  resistance  in  the 
rotor  circuit.  Squirrel-cage  rotors  having  low  resistance,  are 
usually  started  free  and  come  quickly  up  to  speed,  after  which 
the  load  may  be  applied  by  means  of  the  friction  clutch  or  shifted 


PI 


'SI 


AND      <f> 

i.  ~  PI. 


p.  2    MXV. 


Fig.  148. 


belting.  For  induction  motors  intended  to  start  under  load,  how- 
ever, a  starting  device  is  required  which  consists  in  part  at  least 
of  a  rotor  resistance.  The  explanation  of  this  principle  is  as 
follows : 

As  in  the  transformer,  the  primary  flux  of  phase  I,  ^  and  the 
magnetizing  current  causing  it  lag  90°  behind  the  primary 
e.  m.  f.  E/i ,  according  to  Fig.  148.  The  secondary  e.  m.  f.  E^i 
then  lags  90°  behind  $,,.  If  resistance  of  the  secondary  circuit 
is  zero  and  inductance  alone  is  present,  the  secondary  current, 
!„,  lags  90°  behind  the  secondary  e.  m.  f. ,  and  therefore  1 80° 


156 


THE  MOTOR  AND  THE  DYNAMO 


behind  the  primary  flux.  All  this  refers  to  one  phase  only. 
Meanwhile  the  other  phase  has  been  following  on  90°  behind,  so 
that  the  magnetizing  current  Ip2.mag.  and  the  primary  flux  <£/2  due 
to  it  are  90°  behind  3^  and  90°  ahead  of  Islt  Under  these  cir- 
cumstances, the  primary  flux  of  phase  i  can  cause  no  rotation, 
being  in  such  a  direction  as  to  induce  current  ISI  instead 
(Lenz's  law),  and  the  primary  flux  of  phase  2  could  cause 
but  little  torque,  being  zero  when  the  rotor  current  is  a 
maximum,  and  vice  versa.  By  increasing  the  secondary  resist- 
ance we  obtain  the  phase  relation  expressed  by  Fig.  149,  and 


PI. 


s.i 


Fig.  149. 

although  the  secondary  current  is  diminished  by  this  means,  the 
torque  is  increased. 

The  rotor  resistance,  which  gives  the  greatest  torque  at  stand- 
still is  one  such  that  the  resitance  and  reactance  are  equal,  giving 
a  power  factor  angle  of  45°.  A  greater  resistance  than  this 
increases  the  power-factor  causing  1^  to  swing  in  closer  to  Er 
in  Fig.  149,  but  at  the  same  time  reducing  the  rotor  current  so 
much  that  the  torque  is  again  decreased. 

For  the  formula  of  the  induction  motor  and  detail  calculations 
of  its  construction,  the  reader  is  referred  to  "Electric  Motors," 
by  Crocker  and  Arendt,  published  by  Van  Nostrandt  Co.,  1910. 


ALTERNATING    CURRENT    MACHINERY 


157 


(j)  Starters  for  Polyphase  Induction  Motors. 

For  starting  polyphase  induction  motors,  the  device  most 
largely  used  is  a  resistance  in  the  rotor  circuit.  In  small  motors, 
up  to  about  15  horse-power,  it  may  be  contained  in  the  space 
within  the  rotor  surrounding  the  shaft.  One  end  of  the  shaft  is 
hollow,  and  a  handle  protruding  through  this  operates  a  sliding 
contact,  which  gradually  cuts  out  the  resistance,  while  bringing 


Fig.  150.— Forms  of  rotor  with  stator  for  polyphase  induction  motors.     G.  E.  Co. 

the  motor  up  to  speed.  Resistance  coils  so  placed  are  never 
heavy  enough  to  be  more  than  starting  resistances.  On  larger 
machines  and  in  cases  where  the  device  is  not  only  for  starting, 
but  is  a  means  of  obtaining  variable  speed,  the  rotor  windings 
are  brought  out  to  rings  with  contractors,  and  wires  lead  from 
these  to  external  resistances.  The  rotor  is  usually  wound  three- 
phase,  Y  connected,  and  the  resistances  are  varied  by  means  of 
a  controller.  Fig.  150  shows  these  various  types  of  rotor.  Fig. 
150  also  shows  usual  startor  winding. 


158 


THE  MOTOR  AND  THE  DYNAMO 


Fig.  151  shows  curves  obtained  with  various  resistances  in 
the  rotor  circuit.  These  are  so  chosen  with  respect  to  the  rotor 
circuit  that  the  total  resistance  will  enable  the  motor  to  develop 
its  maximum  torque  at  standstill  (100  per  cent,  slip),  and  then 
as  the  speed  increases,  the  resistance  may  be  reduced  by  the  con- 
troller in  such  amounts  as  to  maintain  a  comparatively  constant 


0        10        20       30       40       SO       60        70       dO       90       100 

Per  Cent  Synchronism 

Fig.  151. — Curves  of  polyphase  induction  motor.    G.  E.  Co. 

torque.  The  heavy  line  represents  the  portions  of  the  curves  used 
during  this  process  of  starting  under  load.  When  the  resistance 
coils  are  sufficiently  heavy  to  stand  the  current  without  undue 
heating,  this  starting  device  may  be  employed  to  obtain  variable 
speeds  of  operating.  The  usual  practice  is  to  build  these  rheo- 
stats for  intermittent  use  from  zero  to  half  rated  speed  of  the 
motor,  and  for  constant  service  for  half  to  full  speed.  Figs. 
152  and  153  represent  controller  and  rotor  resistance. 


ALTERNATING    CURRENT    MACHINERY  159 

Another  means  of  reducing  the  current  at  starting  is  to  lessen 
the  e.  m.  f .  applied  to  the  stator.  This  is  usually  accomplished 
by  means  of  an  auto-transfer  in  the  stator  circuit.  Figs.  154  and 
155  represent  one  method  of  connection  for  a  three-phase  circuit. 


Fig.  152.— Controller  for  polyphase  induction  motor.    G.  E.  Co. 

When  the  motor  has  attained  the  full  speed  possible  under  these 
conditions,  the  stator  is  thrown  directly  onto  the  line  by  means 
of  the  controller  or  a  double  throw  switch.  This  device  is 
applicable  to  both  squirrel-cage  motors  and  those  with  wound 
rotors,  but  is  clumsy  and  costly  when  made  for  large  machines. 


i6o 


THE;   MOTOR  AND  THE  DYNAMO 


The  Bell  Electric  Motor  Co.  of  Garwood,  N.  J.,  have  very 
recently  placed  upon  the  market  a  new  type  of  polyphase  motor 
known  as  their  "Compensated  type."  The  characteristics  of  this 
machine  in  operating  are  very  interesting.  There  are  two  sep- 
arate windings  on  the  armature  core.  One  is  a  progressive  wind- 
ing somewhat  similar  to  a  four  pole  direct  current  armature  wind- 


Fig-  153.— Rotor  resistance  of  polyphase  induction  motor.    G.  E.  Co. 

ing,  leads  of  which  are  brought  out  to  commutator  segments. 
Upon  this  winding  but  insulated  from  it  there  is  placed  a  squirrel- 
cage  winding  of  high  resistance,  which  is  entirely  short-circuited 
upon  itself.  These  two  windings  of  high  resistance  give  the  motor 
considerable  torque.  After  the  armature  has  arrived  at  a  pre- 
determined speed,  the  commutator  segments  are  short-circuited 


ALTERNATING    CURRENT    MACHINERY 


161 


Generator 


Fig.  154.— Starting  compensator.     Connections  for  three-wire  two-phase.     G.  E.  Co. 


-  J55- — Starting  compensator.     Kxterior.    G.  E.  Co. 


162 


THE  MOTOR  AND  THE  DYNAMO 


by  a  centrifugal  device.  This  throws  in  the  entire  copper  of 
the  armature,  and  we  then  have  what  are  practically  two  separate 
squirrel-cage  armatures  that  are  probably  in  inductive  relation 
to  each  other,  but  are  not  electrically  connected.  In  starting,  all 
that  is  necessary  for  operation  is  an  ordinary  knife-switch,  no 
compensators,  starting  boxes  or  resistances  of  any  kind  being 
employed.  These  motors  will  bring  their  full-load  torque  up  to 
speed  on  twice  full-load  current.  The  power  factor  and  effi- 
ciency on  all  sizes  is  extremely  high.  These  motors  are  en- 
dorsed by  electric-lighting  companies,  as  they  do  not  seriously 
interfere  with  the  line  voltage,  when  starting. 

(k)  The  Single-Phase  Induction  Motor. 

Motors  of  the  induction  type  in  sizes  up  to  fifty  horse-power 
are  now  manufactured  for  operating  on  the  single-phase  current. 
If  built  in  all  other  respects  like  a  poly-phase  motor,  such  a 


TO  LINE. 


«—  REVERSE 


TO  LI  ML 

Emerson  CatM*  JJ3 

Fig.  156.— Emerson  phase-splitting  device.    Diagram  of  connections. 

machine  would  have  only  an  oscillating,  not  a  rotating,  field,  and 
therefore  no  starting  torque,  and  until  a  certain  speed  is  attained 
it  could  exert  no  torque  at  all.  Such  a  motor,  however,  if  once 
brought  up  to  a  sufficient  speed,  will  fall  into  step  with  its  oscil- 
lating field,  and  may  then  be  loaded  like  any  other  induction 
motor.  If  the  rotor  slots  are  comparatively  near  together,  a 
motor  of  this  type  may  be  started,  running  light,  by  hand,  a  few 
quick  turns  by  means  of  the  pulley-belt  being  sufficient.  When 


ALTERNATING   CURRENT    MACHINERY  163 

the  machine  has  attained  full  speed,  the  load  is  applied  by  a 
sliding  belt,  or  a  friction  clutch,  etc.  Other  devices,  however, 
for  starting  these  motors  are  enumerated  below. 

The  Emerson  motor  employs  a  small  secondary  stator  winding 


157-—  Single-phase  stator,  showing  shading-coils. 


of  high  inductance,  in  which  the  current  lags  about  90°  behind 
that  in  the  main  stator  winding,  thus  producing  a  rotating  field 
flux  similar  to  that  in  the  two-phase  motor.  This  is  known  as 
the  split-phase  method.  The  secondary  winding  is  cut  out  auto- 
matically, when  the  motor  has  attained  full  speed.  See  Fig.  156 


164 


THE:  MOTOR  AND  THE)  DYNAMO 


For  small  fan  motors,  a  simple  phase-splitting  device  known 
as  the  shading  coil  is  frequently  used.  It  consists  of  a  copper 
band  placed  about  one  tip  of  each  pole-piece  as  shown  in  Fig. 
157.  It  has  the  effect  of  causing  the  flux  from  the  pole-tip 
to  be  in  a  different  phase  from  the  main  part  of  the  flux,  and 
thus  to  create  a  torque  sufficient  to  start  the  motor. 

One  of  the  best  starting  devices  for  single-phase  induction 
motors  is  that  of  the  General  Electric  Co.  represented  in  Fig.  158. 
The  stator  is  wound  as  if  for  three-phase,  one  of  the  terminals 
being  excited  through  a  "condenser-compensator,"  which  has  a 


To  motor. 

Fig.  158.— Condenser-compensator  starter. 

phase-splitting  effect,  reducing  the  angle  of  lag  for  a  portion  of 
the  current.  One  form  of  this  starting-box  requires  the  handle  to 
be  pressed  down  for  starting.  When  the  motor  has  attained  full 
speed,  the  handle  is  released,  and  a  spring  lifts  it  clear  of  the 
contact  of  the  compensator  circuit. 

The  Repulsion  Motor. — A  form  of  single-phase  induction  motor 
which  develops  a  considerable  starting-torque  is  made  with  a 
rotor  in  all  respects  like  the  armature  of  a  direct-current  machine 
with  a  commutator  at  its  end,  the  same  as  that  of  a  single-phase 
induction  motor.  The  brushes  are  placed  at  an  angle  and  per- 
manently short-circuited,  as  represented  in  Fig.  159.  The  usual 


ALTERNATING   CURRENT    MACHINERY  165 

method  of  explaining  the  action  of  this  motor  is  to  consider  the 
alternating  flux,  3>,  which  is  produced  by  the  stator  current,  as 
made  up  of  two  components.  Of  these  the  component  3>,  acts 


Fig.  159.— Repulsion  motor. 

like  a  transformer  flux,  inducing  current  in  the  short-circuited 
armature;  and  the  component  ®,n  acts  like  the  ordinary  field  flux 
of  any  motor,  exerting  a  torque  on  the  armature  inductors. 

One  of  the  chief  uses  of  this  type  of  motor  is  that  it  furnishes 
a  starting  device  for  the  single-phase  induction  motor,  when 
required  to  exert  a  large  starting  torque.  The  first  manufac- 


Fig.  160. — Bell  short-circuiting  device. 

turers  to  develop  this  method  of  starting  were  the  Wagner  Elec- 
tric Co.,  but  this  type  of  machine  is  now  manufactured  by  others. 
The  method  consists  in  applying  to  the  armature  of  the  ordinary 
repulsion  motor  a  short-circuiting  ring,  which  is  pushed  into  place 


i66 


THE  MOTOR  AND  THE  DYNAMO 


Fig.  161.— 5  h-p.  single  phase  motor. 


Test  Curve  of  5  H.  P.,  1800  R    P    M.,  220  Volts,  60  Cycles  Bell  High  Efficiency  Single  Phase  Motor 

Fig.  162. 


ALTERNATING    CURRENT    MACHINERY 


I67 


against  the  commutator  bars  when  the  motor  has  attained  nearly 
synchronous  speed.  This  is  done  automatically  by  a  centrifugal 
device,  attached  to  the  shaft.  When  the  motor  is  at  rest,  this 
ring  is  removed  by  a  spring.  Figs.  160  and  161  show  the  method 
employed  by  the  Bell  Electric  Motor  Co.,  and  Fig.  162  is  a  set  of 
curves  representing  the  performance  of  one  of  their  machines. 

(k)  Practical  Remarks  Regarding  Induction  Motors. 

In  specifying  an   induction  motor  for  any  definite  work,  the 


Fig.  163.— 1,400  k.w.  motor-generator  set  with  n,ooo-volt  induction  motor.    G.  E.  Co. 

engineer  has  to  consider  the  two  following  points :  First,  the 
machine  must  be  large  enough  to  develop  the  full  torque  that 
will  be  demanded  of  it,  which  can  in  most  cases  not  exceed  more 
than  200  per  cent,  of  the  rated  full  load  torque  of  the  machine; 
and  second,  the  motor  must  not  be  larger  than  actually  necessary, 
12 


1 68 


THE   MOTOR  AND  THE  DYNAMO 


because  induction  motors  act  with  low   power   factor   and  low 
efficiency,  when  the  load  is  much  below  rated  value. 

Induction  motors   made   a   few   years   ago   were  considerably 
larger  of   frame  than  those  now  manufactured  with  the  same 


Fig.  164.— Motors  geared  to  a  lo-foot  vertical  boring  mill.     Main  motor  15  h-p. 
Elevating  motor  3  h-p.     Westinghouse  Co. 

power  rating.  The  present  practice  of  manufacturers  is  to  have 
stated  sizes  of  rotor  and  stator  punchings  of  sheet  iron,  and  to 
make  one  size  punching  serve  for  two  or  more  sizes  of  motor, 
according  to  the  number  of  such  laminations  used.  In  this  type 
of  machine  the  open  form  furnishes  good  ventilation,  the  lamina- 


ALTERNATING   CURRENT    MACHINERY  169 

tions   being  bound  together  by   horizontal   rods   secured   to   the 
end-plates.     See  Fig.  163. 

Fig.    164   shows   some   applications   of   the   induction   motor. 

(1)  The  A.  C.  Series  Motor. 

The  action  of  the  alternating-current  series  motor  and  its 
characteristic  curves  are  not  far  different  from  those  of  the 
direct-current  series  motor.  Owing  to  the  low  flux  density  of 
alternating-current  machinery  generally,  the  alternating-current 
series  motor  weighs  considerably  more  and  is  larger  than  its 
direct-current  counterpart. 

In  the  action  of  the  alternating-current  series  motor,  the  fol- 
lowing peculiarities  are  to  be  noted: — 

The  iron  losses  are  much  larger  than  in  the  direct-current 
machine,  owing  to  the  alternating  flux  in  not  only  the  armature 
core  but  also  the  field.  On  this  account,  the  field  core  has  to 
be  laminated,  which  considerably  increases  its  size. 

Besides  the  c.  e.  m.  f .  always  present  in  a  rotating  armature, 
there  is  developed  in  the  armature  windings  another  e.  m.  f .  by 
transformer  action  from  the  alternating  field-flux.  This  e.  m.  f . 
neither  aids  nor  opposes  the  counter  e.  m.  f.,  the  division  of  arma- 
ture inductors  in  respect  to  direction  of  this  e.  m.  f.  being  at 
right  angles  to  the  axis  of  commutation,  so  that  one  half  counter- 
acts the  other.  This  transformer  e.  m.  f.,  however,  greatly 
increases  the  tendency  to  spark  in  the  coil  short-circuited  by  the 
brushes. 

The  current  in  the  short-circuited  coils  is  sometimes  reduced 
by  inserting  a  high  resistance  where  the  armature  coils  are  con- 
nected to  the  commutator  bars. 

Another  device  found  in  series  alternating-current  motors  is 
the  compensating  winding.  It  consists  of  several  turns  of  wire 
let  into  grooves  in  the  pole  faces,  and  serves  to  reduce  armature 
reaction  and  the  self-induction  of  both  field  and  armature  circuits. 

Figs.  165  and  166  enable  the  reader  to  make  a  comparison  of 
the  characteristics  of  the  alternating-current  and  the  direct-cur- 
rent series  motor. 


170 


THE  MOTOR  AND  THE  DYNAMO 


It  is  sometimes  necessary  to  operate  alternating-current  series 
motors  on  a  direct-current  circuit,  as  in  the  case  of  electric  loco- 


'•^Q£± 


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Fig.  165.— Direct-current  motor.    G.  E.  Co. 


motives  operating  on  two  differently  equipped  roads.     Small  fan 


ALTERNATING   CURRENT    MACHINERY 


motors  and  the  like  also  capable  of  operating  on  either  alternating- 
current  or  direct-current  circuit  are  now  manufactured. 


Fig.  166.— Alternating-current  motor.    G.  E.  Co. 


INDEX 

PAGE 

Absolute  unit  of  current 5 

Absolute  unit  of  e.  m.  f. 5 

Alternating-current,  definition  of 105 

generation  of .    14,  120 

measurement  of 108-1 13 

Alternating  e.  m.  f 107 

A.  C.  circuits  in  parallel 119 

A.  C.  circuits  in  series 1 18 

ALTERNATORS  : 

Compound 130-131 

Operation  in  parallel 136 

Single-phase 14,  120,   129 

Polyphase 120-125 

Voltage  regulation  of 125-131 

Angles  of  lag  and  lead 50-107 

Arc-light  dynamos — 64 

Armature,  construction  of 23-33>  I24 

winding 25-33,  124 

Armature  characteristic 53 

Armature  reaction  in  generators 49 

in  motors 50 

Auto-transformer 150 

Balancer 97 

Bearings 37 

Booster 63 

Brush-holders 35-37 

Brush  generator  65 

Brushes    35,  45 

number  of 31,  32 

adjustment  of 50,  35 

Building-up  conditions  for    40,  41 

Building-up  curve 46 

Capacity  in  A.  C.  circuits 112 

Cast  iron,  magnetic  properties 21 

CHARACTERISTIC  : 

Armature 53 

External 51 

Of  alternators 1 25 

Of  D.  C.  generators .' 47-55 

Of  A.  C.  motors 132,  153,  154,  158,  166,  171 

Of  D.  C.  motors 74-78,  83 


174  INDEX 

PAGE 
Circuits,  A.  C.  in  parallel 119 

A.  C.  in  series 118 

Clock  diagrams 107 

Collector  rings 14 

Commutation,  axis  of 50 

Commutator,  care  of 39 

construction  of 33-35 

Compensating  winding 131 

Compensator  for  induction  motors 161,  164 

Compound  D.  C.  generator 54-6i 

Componnd  motor 76-78 

Compounding  of  alternators 130 

of  D.  C.  generators 55 

Constant  current  generator 64 

Constant  voltage  generator 55 

Control  of  generator  voltage 53 

Control  of  motor  speed 79 

Control,  series  parallel 95 

multiple  unit 96 

Controllers  90-97 

Counter-e.  m.  f.  in  motors 15,  66 

Curves  of  A.  C.  current  and  voltage 108 

Curves,  (See  Characteristics) 

Current  in  A.  C.  circuits 119,  121,  123 

Delta  connections 122 

Differential  motor 77 

Drop  of  voltage  in  armature 15,  45,  51 

Drum-type  armature 24 

DYNAMO  :     (See  also  Generator) 

Fundamental  equation 14 

general  construction  of  D.  C. 12-17 

general  construction  of  A.  C 14,  124 

management  and  operation 39,  53 

Eddy-currents 23,  98 

Eddy-current  losses 98 

Edison  three-wire  system 63 

Effective  A.  C.  volts  and  amperes 108 

Efficiency  of  generators 98 

of  motors 73-76,  83,  100,  166,  170,  171 

by  brake  test 74 

by  stray  power  test 98 

Electrical  horse-power 74 


INDEX  175 

PAGE 

Electro-motive  force,  induced 5 

of  generators !4 

in  A.  C.  circuits 109 

Electro-magnetism y 

Equalizer 6r 

Exciter  for  alternators 1 2  r 

External  characteristic 52 

Farad,  definition II2 

Faults  in  armature 42 

Faults  in  connections 40 

Field-core 17-20 

Field-core  losses 9g 

Field-magnets 2  r    ^y 

Field-rheostats 89-91 

Field-windings 15,  21 

Flat  compounding 1-4 

Flux  and  flux  density 4 

Flux  in  solenoid g 

Foucault  currents 2^,  98 

Fundamental  equation  of  the  generator !4 

of  the  motor I5 

General  formula  of  dynamo .-.  .     ™ 

GENERATORS : 

Alternating-current l^  I20 

Direct-current I2 

Characteristics  of 47-55 

Compound 54 

Edison  three-wire 63 

Efficiency  of I00 

In  parallel,  A.  C. I^ 

In  parallel,  D.  C.  58-62 

Shunt 51-54 

Series 64 

Troubles  of 40 

Grounds 42 

H  and  B  curves .    9,  47 

Harmonic  current IOc 

Henry,  definition  of ITQ 

Hunting  of  A.  C.  machines j^y 

Hysteresis 9>  Io 

Induced  electro-motive  force 5 

Inductors  on  armature 30 

Inductance,  definition  of I  lo 

effect  of  in  circuit 1 10 


176  INDEX 


PAGE 

Induction  motor 150 

polyphase 151,  156 

single  phase 162 

speed  control  of 158 

starting  devices 157-164 

Inductor  alternator 129 

Insulation  tests 42 

Iron  losses  in  dynamos 98 

Iron,  magnetic  characteristics 9 

Interpole  motor .  •  •  • 83 

Lagging  current 107 

Lap-winding 31 

Leading  current 112 

Lead  of  brushes 50 

Leakage,  magnetic 49 

Lenz's  law 3,7 

Lines  of  force i 

Load  losses  in  dynamos  and  motors - 98 

Losses  in  dynamo  machinery 98 

Magnetic  circuit  in  dynamos  21 

Magnetic  field  in  solenoid 8 

Magnetic  flux 4 

hysteresis 10 

permeability 10 

properties  of  materials 21 

reluctance 8 

Magnetism,  residual 41 

Magnetization  curve 48 

Magneto-motive  force 8 

MOTORS  : 

Alternating-current 132-136,  150-171 

Direct-current 67-97 

Characteristics  of 75,  76,  154,  166,  171 

Compound 78 

Induction 150-169 

Repulsion 164 

Series 72,  76,  77,  171 

Shunt 72,  75 

Speed  control 79-90 

Starting-boxes  for 90 

Synchronous 132 

Torque  of 70 

Motor-generator 97 

Non-inductive  circuit 1 16 


INDEX  177 

PAGE 

Ohm's  law 1 1 

Oscillograph 117 

Over-compound  generator 55 

Parallel  operation  of  alternators 136 

of  D.  C.  generators 58-62 

Permeability 10 

Phase  in  alternating-current 120-123 

Poles  of  dynamo 12,  17 

Polyphase  alternator 124 

Polyphase  current 120-123 

Power  factor • 114 

Power  in  A.  C.  circuits 114,  116,  117 

Power  losses  in  dynamo  machines  •  •  •    98 

Power  of  motors 70 

Railway  motors 93 

Railway  motor  control 95-97 

Reactance,  definition  of no 

effect  of in 

Regulation  of  generator  voltage 53,  126 

of  motor  speeds 71 

of  voltage  in  A.  C.  generators 126,  131 

Reluctance,  definition  of 9 

Residual  magnetism 41 

Resistance,  effect  of 1 1 

measurement  of i  r 

Resistance  in  rotor  of  A.  C.  motors 155 

Rheostats  for  dynamo  fields 90 

Rheostat  control  of  motors  71,  86,  92,  158 

Rotary  converter 140 

Rotor  of  induction  motor 152,  157 

Saturation,  magnetic , 47 

Self-induction no 

Series  generators 64 

Series  motors 72,  93,  169 

A.  C.  and  D.  C.  compared 170,  171 

Series  motor  starters 95 

Shunt  generators  , 51-54 

Shunt  motors . . . 72,  76,  77,  151 

Single  phase  motors 162-167,  ^9 

Single  phase  motor  starting  devices 163-165 

Slip  in  induction  motors 153 

Sparking,  cause  of 56 

prevention  of,  in  motors 82 

Speed  control  of  motors 79,  90,  96,  158 


178  INDEX 

PAGE 

Split-phase  starting  device 163 

Squirrel-cage  rotor 152 

Starting-boxes  (See  Controllers). 

Steel,  magnetic  properties  of 21 

Steinmetz's  formula 10 

Step-up  and  step-down  of  A.  C.  e.  m.  f. 147 

Synchronizing ,    133 

Synchronous  converter  (See  rotary  converter). 

Synchronous  motor 132 

Synchronous  reactance  of  alternator 129 

Synchroscope 137 

Three-phase 122 

Tyrrell  regulator 131 

Traction  motors 93-97 

Transformers 146-150 

Transformer  losses 148 

Two-phase 120 

Variable  speed  motors 79-90,  158 

Voltage  in  A.  C.  circuits 118,  122,  123 

Voltage  control  of  generators 53,  55,  130 

Voltage  regulation  of  generators 126 

Voltmeters 115 

Wattmeters 117 

Watts  in  A.  C.  circuits 114-117 

Wattless  current 1 16 

Wave-winding 32 

Winding  armatures 25-33,  I24 

Wrought  iron,  magnetic  properties 21 

Y-connection 122,  123 


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178  INDEX 

PAGE 

Split-phase  starting  device 163 

Squirrel-cage  rotor 152 

Starting-boxes  (See  Controllers). 

Steel,  magnetic  properties  of 21 

Steinmetz's  formula 10 

Step-up  and  step-down  of  A.  C.  e.  m.  f.  147 

Synchronizing 133 

Synchronous  converter  (See  rotary  converter). 

Synchronous  motor 132 

Synchronous  reactance  of  alternator 129 

Synchroscope 137 


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